Security forces have attempted to storm a mosque in the southern Syrian city of Daraa, reportedly killing at least six people.
The government, however, blamed the violence on an "armed gang", according to the state-run SANA news agency.
Residents said heavy gunfire was heard near the Omari mosque in the early hours of Wednesday in the city, which has been the scene of anti-government protests since Friday.
"It seems that security forces may be trying to storm the complex. It is not clear because electricity has been cut off. Tear gas is also being used," one resident told the Reuters news agency.
Protesters calling for political freedoms and an end to corruption had said earlier that they were going to remain in the mosque until their demands were met.
Mohammed Al Abdallah, an exiled Syrian rights activist living in the US, said he had been communicating with people in Daraa who told him that a doctor was among those killed.
"Security forces opened fire, they used bombs and live ammunition," he told Al Jazeera. "There are many injuries, including women and children."
He said electricity had been cut off before the attack and that security forces were preventing ambulances from entering the centre of Daraa, where the mosque is located.
Al Jazeera could not immediately confirm the reports and mobile phone network appeared to be disrupted in Daraa.
Rula Amin, Al Jazeera's correspondent in Damascus, said the incident was a "very serious development" that could trigger protests, and the prospect of further violence.
She also said the use of violence against protesters indicated the government had abandoned its earlier offer of dialogue.
"With the killing of these six people overnight, it seems that the government is opting for the choice of using force to crack down on the protesters not dialogue as it had indicated," she said.
She reported that 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 came a day after the United Nations' human rights chief called for a probe into a weekend crackdown, when several protesters were killed in Daraa.
Navi Pillay, the UN High Commissioner for Human Rights, called on Syrian authorities to carry out a transparent probe into the crackdown and 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.
"People have the legitimate right to express their grievances and demands to their government, and we urge the Syrian government to enter into a broad, meaningful dialogue with the protesters in an attempt to address those grievances," he said.
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.
Wednesday's incident brings to 12 the number of people reportedly killed by security forces since the start of the demonstrations on March 18, including an 11-year-old boy who died after inhaling tear gas on Monday.
A Syrian official told the AFP news agency that the governor of Daraa had been sacked following the killings.
The official, who wished to remain anonymous for fear of retribution, said Faisal Kalthoum was fired from his position on Tuesday.
Al Jazeera's Amin said that Kalthoum had not officially been sacked but had not been seen in public for the past three days.
She said the latest round of protests were "unprecedented", but had not yet spread to other cities.
"It's a very serious challenge to the regime here. These are unprecedented demonstrations, they have been going on since Friday, and this has not happened in Syria for a long, long time. And it seems people are not detered by heavy security deployment there.
"So far, most of the protests have been confined to Daraa and the villages around it. It has not spread to Damascus yet, or elsewhere in Syria, despite calls by opposition and activists for people to take to the streets."
Malik al-Abdeh, a Syrian journalist and member of the Movement for Justice and Development, told Al Jazeera that President Assad "should be extremely worried".
"When he inherited the presidency from his father, he promised reform. And for the last 11 years he's used the language of reform, but on the ground there hasn't been any real, meaningful political reform," he said.
"So the Syrian people are very angry. They gave him the benefit of the doubt, they stood by him when the US tried to isolate him following the assassination of [then Lebanese prime minister] Rafik Hariri, and their loyalty was paid back by blood, by bullets, by the massacre that we saw in Daraa this morning."
He said that while protesters initially wanted reform within the system, they would now very likely push for outright change in the country's leadership.
"Well, to begin with, when the protests began ... it was very clear that the people wanted reform. They wanted to apply pressure on the government to press ahead with meaningful political reforms. However, a week on, and now following the deaths of 12 people in Daraa and following the arrest of over 1,000 in Syria, I think the demands for regime change will become ever higher.
"Certainly the people in Daraa are very angry and they're calling for revenge, and I'm sure that a large section of them legitimately will now be calling for regime change, because Bashar al-Assad seems to be following in the footsteps of his friend [Libyan leader] Muammar Gaddafi by ordering his soldiers to fire on innocent, unarmed protesters."