At least seven people were killed Wednesday when Syrian security forces fired on a pro-democracy protest in the central city of Homs, according to activists.

Security and army forces were carrying out a massive-scale crackdown in several areas of the dissident city, said the Local Coordination Committees of Syria, an activist group.

Explosions accompanied by heavy gunfire were heard in several neighborhoods of Homs, the group added.

"Armored vehicles have entered the city as helicopters are hovering in the sky," broadcaster Al Jazeera quoted an activist as saying.

Snipers stationed on top of buildings in Homs were firing indiscriminately, he added.

The clampdown comes hours after the state news agency SANA reported that the Syrian government had asked the head of the Arab League Nabil al-Arabi to postpone a visit that was originally scheduled for Wednesday.

At least 2,200 people have been killed since pro-democracy protests started in Syria in mid-March, human rights advocates say.