Activists say Syrian security forces have fired tear gas at thousands of protesters who rallied in Homs on Tuesday as Arab League monitors visited the flashpoint city to track the government's crackdown on dissent.

The Syrian Observatory for Human Rights says about 70,000 anti-government protesters were trying to rally in the city's center.

It is not clear if Arab League monitors were still meeting with the region's governor when the incident took place. Syrian state-run media reports say the monitors wrapped up their first day of observations in Homs but the state media did not give a detailed account.

Fifty observers and 10 other Arab League officials arrived in Syria on Monday to monitor the government's promises to withdraw security forces from cities, release political prisoners and halt violence against civilians.

The Arab League says the observers are also to visit several other Syrian cities, including Idlib, Hama and Daraa.

The United Nations estimates 5,000 people have been killed since March in violence linked to the uprising against Syrian President Bashar al-Assad.

Homs has been a center of the anti-government unrest.

Activists say the security forces killed at least 30 people in Homs on Monday. They say government forces had intensified their attacks on city districts in recent days, but then pulled back before the monitors' arrival.

The state-run news agency SANA reports “armed terrorists” blew up a gas pipeline in Homs on Tuesday. It also says “terrorists” clashed with authorities near the Turkish border.

Syria say armed terrorists are driving the revolt. It accuses them of killing 2,000 security personnel since March.

Syrian authorities agreed to the Arab League observers under international pressure and threats of Arab sanctions.

The plan requires Syrian authorities to give the monitors freedom of movement everywhere except sensitive military sites.