Around 20,000 Syrians chanting freedom slogans marched on Thursday at the funerals of nine protesters killed by security forces in the southern city of Daraa, witnesses said.
"The blood of martyrs is not spilt in waste!" they chanted in Daraa's southern cemetery.
The nine were among at least 25 people shot dead by security forces on Wednesday, residents said.
A witness told Al Jazeera that more than 100 people were killed.
He said many people have gone missing and bodies have been dragged away from the streets. The town was in chaos, he said.
A hospital in Daraa had said earlier that it had received the bodies of at least 25 protesters who died in confrontations with security forces.
"We received them at 5pm local time on Wednesday (1500 GMT). They all had bullet holes," the official told Reuters news agency on Thursday.
The AP news agency quoted an activist as saying that some residents of the southern town are holding a sit-in to protest the killings.
The activist, who is in contact with residents in Daraa, said the situation is still tense, with a heavy presence of security forces in the streets.
He said dozens of people were holding the sit-in in the al-Mahata neighborhood near the city centre.
Inspired by the wave of pro-democracy protests around the region, Daraa residents have held protests since last week.
Earlier, human rights activists said at least 15 people have been killed in Daraa.
Residents said security forces shot and killed six people including a doctor who was giving aid to the injured at the Omari mosque, where most of the protests took place.
A rights activist also told AFP news agency that security forces had opened fire on mourners attending the funeral of those killed in Daraa.
Call for Friday protests
Meanwhile, pro-democracy demonstrators in Syria have called for mass protests across the country on Friday.
Activists used social-networking sites to call for the protests, which they dubbed as "Dignity Friday."
Al Jazeera's Rula Amin, reporting from Damascus, said violence broke out in Daraa when residents from other towns clashed with security forces as they tried to enter it 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.
'Need for radical change'
Checkpoints have been set up by security forces at all entry points to the city. There was also no mobile phone network coverage in Daraa on Wednesday.
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.
Later on Wednesday, state television showed what it said were pictures of a weapons stockpile inside the Omari mosque, including pistols, shotguns, grenades and ammunition.
A Syrian official told the AFP news agency that the governor of Daraa had been sacked following the killings.
Authorities have arrested a leading campaigner who had supported the protesters, the Syrian Observatory for Human Rights said on Tuesday. It said Loay Hussein, a political prisoner, was taken from his home near Damascus.
A number of Syrian towns and cities saw demonstrations in recent days despite the country's emergency law which bans protests that has been in place since 1963.
The United Nations, France and the United States condemned the violence. Ban Ki-moon, the UN secretary-general, 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.