Human rights activists say at least 15 people have been killed in the Syrian town of Daraa, the focal
point of almost a week of anti-government protests.
Activists and residents said security forces opened fire on protesters outside the Omari mosque early on Wednesday, after hundreds of people had gathered overnight to prevent police from storming it, and that shooting had continued sporadically over the course of the day.
A rights activist also told AFP news agency that security forces had opened fire on mourners attending the funeral of those killed in Daraa.
Al Jazeera's Rula Amin, reporting from Damascus, said that fighting broke out when residents from other towns clashed with security forces as they tried to enter Daraa to help residents there.
A youth activist in the Syrian capital, who remains anonymous, told Al Jazeera that his contacts in Daraa said that "dozens of people" had died in clashes.
"Many there want to take down the government, and want more freedoms." he said.
Our correspondent said there was a heavy security presence in Daraa, with the army, anti-terror police and riot police all deployed in the city. Journalists are not being allowed to visit the city, and several of those who attempted to do so last night had their equipment confiscated by authorities.
Checkpoints have been set up by security forces at all entries to the city.
Syria's state-run television station reported that an "armed gang" attacked an ambulance at the Omari mosque, killing four people.
The victims were a doctor, a paramedic, a policeman and the ambulance driver, according to SANA.
The security forces who were near the area intervened, "hitting some and arresting others," the report said, without elaborating.
Later in the day, state television showed what it said were pictures of a weapons stockpile inside the Omari mosque, including pistols, shotguns, grenades and ammunition.
The violence was condemned by Ban Ki-moon, the UN secretary-general, who called for "a transparent investigation into the killings".
A spokesman for the US state department said Washington was alarmed by the situation and urged Syrian authorities to "exercise restraint and to refrain from violence".
"We are deeply concerned by the Syrian government's use of violence, intimidation and arbitrary arrests to hinder the ability of its people to freely exercise their universal rights. We condemn these actions," said Mark Toner.
On Tuesday, Navi Pillay, the UN High Commissioner for Human Rights, urged Syrian authorities to halt the excessive use of force.
"The government should carry out an independent, transparent and effective investigation into the killings of the six protesters during the events of 18 and 20 March," Rupert Colville, a spokesman for Pillay, said on Tuesday.
"We are greatly concerned by the recent killings of protesters in Syria and reiterate the need to put an immediate halt to the excessive use of force against peaceful protesters, especially the use of live ammunition."
Colville said that the use of excessive force was a "clear violation of international law" and that perpetrators could be prosecuted.
Demonstrations have been held in a number of Syrian cities in recent days despite the country's emergency law, which bans protests and has been in place since 1963.
A Syrian official told the AFP news agency that the governor of Daraa had been sacked following the killings.