Twenty people were killed at rallies across Iran on Tuesday, as opposition activists demontrated against the disputed results of last week's presidential elections, according to various media reports.

Iran's defeated presidential candidate Mirhossein Mousavi urged his supporters not to attend a planned rally in Tehran on Tuesday in light of the violence, his spokesman said.

"Mousavi ... urged his supporters not to attend today's rally to protect their lives. The moderates' rally has been canceled," the spokesman said.

Meanwhile, Iran's top legislative body said Tuesday it was ready to recount the votes in the election, which was won by hardline incumbent Mahmoud Ahmadinejad.

Iran's Guardian Council said a recount of disputed ballot boxes may lead to changes in candidates' tally, according to the television report.

Mousavi has appealed to the council for the election to be annulled, but the Guardian Council ruled out that option.

"Based on the law, the demand of those candidates for the cancellation of the vote, this cannot be considered," council spokesman Abbasali Kadkhodai told state television.

Also on Tuesday, a leading Iranian reformist, Mohammad Ali Abtahi was arrested, state television said without giving further details. Abtahi is a former vice president who backed Mousavi.

The incidents come one day after gunmen fired on opposition protesters at a massive demonstration in Tehran, killing at least one person.

An Associated Press photographer saw one person shot dead and several others who appeared seriously wounded in Tehran's Azadi Square. The shooting came from a compound for volunteer militia linked to Iran's powerful Revolutionary Guard.

Tens of thousands attended the rally, held in support of defeated presidential candidate Mir Hossein Mousavi, In defiance of a government ban. They marched in downtown Tehran on Monday to protest what they said was Iran's rigged election.

"There has been sporadic shooting out there ... I can see people running here," a reporter of Iran's English-language Press TV said in a live call from Azadi Square.

"A number of people who are armed, I don't know exactly who they are, but they have started to fire on people causing havoc in Azadi Square," he said.

Shooting was also heard in three districts of wealthy northern Tehran, residents said.

Members of Iran's security forces have at times fired into the air during two days of the Iranian capital's most violent unrest since the 1979 Islamic Revolution, and used batons to beat protesters who have pelted police with stones.

Several kilometers of a central thoroughfare were packed with crowds who came to hear Mousavi and other pro-reform leaders who back his call for Friday's vote, won by hardline President Mahmoud Ahmadinejad, to be annulled.

State television showed pictures of Mousavi addressing part of the crowd through a megaphone. He told them he was ready in case new elections were called in the Islamic Republic, an aide told Reuters.

"Tanks and guns have no use any longer," chanted the crowd in a deliberate echo of slogans used in protests leading up to Iran's 1979 Islamic revolution.

Calling on Ahmadinejad to resign, they said the election results resembled a "coup d'etat" and chanted "Death to the lying government".

"I just want to show the president that we are not bandits. I want my vote back," said Maryam Sedaghati, a young woman wearing a green headscarf -Mousavi's campaign colour.

Official results gave Ahmadinejad 63 percent of the vote and Mousavi 34 percent, figures which the moderate former prime minister has dismissed as a "dangerous charade".

He has appealed to Iran's top legislative body for the vote to be cancelled.

Wearing green and carrying photographs of Mousavi, the crowd chanted: "Where are the 63 percent who voted for Ahmadinejad?"

As a police helicopter flew overhead, the crowd booed.

They vowed to continue daily protests to keep pressure on authorities. "We fight, we die, we will not accept this vote rigging," they chanted, and also: "By the end of the week, Ahmadinejad will be gone".

The three days of protests and clashes since Iran's election results were declared on Saturday have been the most serious since Iran's Islamic revolution three decades ago.

They follow boisterous election campaigning by Mousavi supporters, who flooded the streets of affluent northern Tehran for nightly rallies ahead of the vote.