Syrian troops have fired on protesters in the city of Homs as Arab League observers toured the area to see whether the regime is keeping its pledge to halt the crackdown on dissent.

In the capital Damascus thousands held prayers for those killed since the uprising began in March. Christian and Muslim religious leaders attended the service, and throngs packed the city's Holy Cross church, its yards and a nearby street.

"Enough killings in our beloved Syria," the country's top Sunni clergyman, Grand Mufti Ahmad Badreddine Hassoun, told the crowd at the prayer service. His son was shot dead in October.

The 165 foreign monitors are supposed to be ensuring that Syria complies with the Arab League plan stipulating the regime stop killing protesters, remove heavy weapons, such as tanks, from all cities, free all political prisoners and allow in human rights organisations and foreign journalists. Syria agreed to the plan on December 19.

However, the crackdown has not stopped and opposition activists say around 450 people have killed by the regime since observers began work on December 21. The British-based Syrian Observatory for Human Rights said security forces shot dead four people around the country and returned the bodies of 10 other people to their families in several Homs neighbourhoods.

The UN estimated several weeks ago that more than 5,000 people have been killed in political violence since March. Since that report, opposition activists say hundreds more have died.

On Sunday, the Arab League repeated its demand for the Syrian government to immediately stop all bloodshed.

It was not immediately clear whether the foreign observers witnessed the regime forces opening fire in the Khaldiyeh neighbourhood of Homs. Several people were reported wounded.

Majd Amer, an activist in Homs, said the shooting started after thousands of protesters surrounded a group of observers, urging them to go to Khaldiyeh, where anti-regime protesters are known to be active. The observers' Syrian escorts wanted to take them to the nearby Abbassiyah neighbourhood, where many regime supporters live, he said.

"Sporadic shooting was heard for a few seconds," he said.