Hundreds of Palestinians stormed the Gaza Strip's border crossing with Egypt on Wednesday, throwing rocks at Egyptian troops and demanding to cross through, witnesses said.

At least six Egyptian police were injured, security and medical sources said.

The Palestinians accused Egypt of reneging on a promise to open the vital crossing, which has been virtually sealed for the past year since the Hamas militant group seized control of Gaza. But Egyptian officials said Hamas had broken the agreement by sending unauthorized people to cross.

The crowd burst past Hamas security forces at the border but was unable to get through the Egyptian side, the witnesses said.

Palestinians began throwing rocks at Egyptian troops, who responded with water cannons. No one made it across the border, and Hamas later restored order, the witnesses said.

Palestinian officials and witnesses said Hamas had beefed up security at the site after the violence and was restoring control, ordering people to leave and forcing the crowd back across the Palestinian border.

The border opened on Tuesday to let sick and injured Palestinians through, as well as Palestinians with residence or work permits in third countries, and to allow Palestinians to return to their homes from Egypt. Officials said then that they planned to keep it open for three days.

But Palestinian officials and witnesses said crowds had become frustrated by the slow pace of entry, with only around 200 Palestinians being allowed through on Tuesday out of the thousands seeking entry, according to Egyptian security sources.

Hamas spokesman Sami Abu Zuhri told Reuters in Gaza, "It (the violence) was a spontaneous act that reflected the state of pressure and suffering the Palestinian citizens are being exposed to in the Gaza Strip... The incident stresses the need to open the Rafah crossing permanently to Palestinian citizens."

An Egyptian security source said Egyptian authorities were threatening a total closure. A Palestinian official in the Gaza Strip said Egypt had closed the border for the day, but would process any Palestinians inside the terminal.

Rafah is the main gateway for Gazans to the outside world, used by the area's 1.4 million people to travel abroad for studies, medical care, jobs and family visits.

In January, Hamas militants blew open the border, allowing thousands of people to cross through until it was resealed about two weeks later. Wednesday's rush on the border raised concerns that people might once again try to breach the border.

Egypt and Israel have kept Rafah closed for the past year to put pressure on Gaza's Hamas rulers. Under a truce mediated by Egypt, Israel has signaled it would reopen the border once Hamas returns the captive Israel Defense Forces soldier Gilad Shalit, it has held for two years. Egypt is currently trying to broker a prisoner swap.

As part of the truce, Egypt had allowed in principle to allow medical patients, students and those with residence in Arab countries to leave over three days, starting Tuesday. At least 6,000 Gazans meeting these criteria registered to cross Rafah.

On Wednesday, thousands were still waiting at the crossing, and by noon, not a single bus had crossed yet. Tensions rose after Egypt said only 200 people would be allowed through, and the crowd started pushing through the first gate on the Gaza side, up to the last gate that separates Gaza and Egypt.

Hamas spokesman Sami Abu Zuhri said his group was sorry for the troubles at the border.

"This reflects the pressure that people are suffering," he said. "We call on Egypt to speed up the process of reopening Rafah terminal in normal way."

In Egypt, security officials and a Palestinian diplomat said the decision to open the crossing this week was made under an agreement that both Hamas and Egypt should coordinate the movement.

An Egyptian official, speaking on condition of anonymity because he was not authorized to talk to the media, said Hamas had violated the agreement by sending people who were not approved to cross.

"This an infringement of the agreement of coordination," the official said. "The opening of the crossing was for humanitarian purposes not for every body."

Israel reopens Gaza crossings

Also Wednesday, Israel reopened crossings on the Gaza Strip border, and for the first time in a year, allowed the transfer of construction material into the coastal territory.

Four crossings between Israel and Gaza reopened on schedule, Israel and the Hamas rulers of Gaza said. The Hamas Interior Ministry said Israel was to allow in fuel, wheat and animal feed, in addition to five trucks of dry cement for the first time in a year. About 26 sick Palestinians were also to enter Israel to receive treatment.

Since a truce with Gaza militants began on June 19, Israel has closed the passages a total of six times in retaliation for the firing of rockets on its population centers. But the Israel Defense Forces said that a lull in rocket strikes Tuesday led to the decision to open the crossings.

"After evaluating the situation where we had a couple of days where rockets were fired, no rockets were fired yesterday and this has enabled us to reopen the crossings," IDF spokesman Major Peter Lerner said.

On Tuesday, Cairo ordered the opening of the Rafah crossing on the Egypt-Gaza border Tuesday, allowing more than 100 people to cross the border. Among them were roughly 50 who needed medical care and another 67 Egyptian citizens who were stranded in the Strip after the closing of the border a few weeks after it was breached by Hamas in January.

Egypt plans on letting additional Palestinians cross from Egypt into Gaza on Wednesday. Since Hamas seized control of Gaza in June 2007, the Rafah crossing has been opened - according to observations by the security establishment - at least 50 times, despite Israel's demand that the crossing remain closed until kidnapped IDF soldier Gilad Shalit is released by Gaza militants.

Senior Palestinian and Israeli officials said they doubt Egypt intends to open the Rafah crossing completely, even after Shalit is released, out of concern that Palestinians would stream into Egypt. It appears that Egypt prefers at this stage to open the crossing intermittently rather than permanently.

A Hamas leader, Ismail Radwan, castigated Egypt on Tuesday for preventing Hamas activists from returning to Gaza. Radwan said it was illogical to be conducting negotiations with the "Zionist enemy" for the release of Palestinian prisoners while Egypt held them.

He also said that Hamas would not retract its demand to include those serving life terms in Israeli jails on its list of prisoners to be exchanged for Shalit.

In response to Israeli objections, Egypt told Israel Tuesday that the opening of the Rafah crossing did not necessarily breach its obligations with regard to the truce it brokered between Israel and Hamas. A senior government official said senior Egyptian officials had told their Israeli counterparts that "the opening of the crossing was done to let off steam that built up over weeks, as is done from time to time."

The Prime Minister's Bureau only comment on the opening of the crossing was to refer the matter to the Defense Ministry.

Abbas urges self-restraint to preserve Gaza cease-fire

Palestinian President Mahmoud Abbas urged self-restraint Tuesday from both sides to preserve the shaky, Egyptian-brokered truce in the Gaza Strip, "despite the fact that there are those who are trying to destroy it."

Abbas was speaking after he and Defense Minister Ehud Barak met briefly Tuesday during a Socialist International parley in Athens, and both indicated their support for peace talks.

The truce was under strain on Tuesday as Israel re-sealed its border crossings with the Gaza Strip after militants fired rockes on the western Negev. Israel closed its crossings into the Strip in response to the fire, and Prime Minister Ehud Olmert warned Palestinian militants that Israel would respond if attacks from the Gaza Strip persisted.

Meanwhile, Noam Shalit, the father of kidnapped soldier, took Olmert and the military and political establishment to task for failing to secure the release of his son after two years. "Conducting successful negotiations over the cost of returning prisoners is a result of the skill of the leader in being creative, determined, effective and experienced in this area, and his willingness to consult with others on the matter. Unfortunately, I did not see that the prime minister had this effectiveness."

Speaking at a conference at Bar-Ilan University on the moral and legal implications of a prisoner swap, Shalit said "Olmert asked Hamas to prepare a list of prisoners to release. Whoever heard of such a thing? It took them nine months to put together the list, which was 'heavy.' Nine months of Gilad's life lost forever."

Shalit was responding to remarks by law professor Yedidiah Stern, who argued that "it is inconceivable for the families of the kidnapped soldiers to play an integral part in determining the price-tag for returning the boys."

Stern called the families' part in the discourse "humiliating," saying that "the State of Israel must determine clear rules on the matter, because now we have paid a high price and in the future it will be higher. Captivity is worse than death. It is a black hole that everything around it is pulled into, but that cannot be what dictates the nation's actions."

Tzvi Regev, the father of kidnapped soldier Eldad Regev, who also took part in the conference, said every soldier has to know that the nation and the flag are behind him. "Despite the reports released about our sons, we have not lost hope," Regev also said.

(Barak Ravid and Jack Khoury contributed to this report)