Syrian security forces opened fire on mourners near a mosque in the flashpoint city of Deraa after a mass funeral for pro-democracy protesters, two witnesses said on Saturday.

Security forces used live ammunition and tear gas to disperse thousands of people who were chanting freedom slogans after assembling near the old Omari mosque in the old quarter of the southern city near the border with Jordan, they said.

Dozens of people have been killed in a wave of protests across Syria against the rule of President Bashar al-Assad.

Accusing the government of commiting a crime against humanity, a Syrian rights group said at least 37 were killed across the country on Friday.

 'What is happening in Syria is a flagrant violation of (human rights),' the National Organisation for Human Rights  said in a statement.

'Syrian security committed (in Deraa) what could be called a crime against humanity ... It fired indiscriminately on protesters and killed and wounded tens of them.'

After Friday's bloodshed, Syria's interior ministry warned it would not tolerate breaches of the law and would deal with 'armed groups', state news agency SANA said on Saturday.

Activists were concerned this could herald a harsher crackdown. Inspired by Arab uprisings that began in Tunisia and Egypt, popular demonstrations calling for greater freedom have shaken Syria.

Assad has responded with a blend of force against protesters, gestures towards political reform and concessions to conservative Muslims including closing Syria's only casino.

In the early hours of Saturday, Syrian security forces used live ammunition to disperse a pro-democracy protest by hundreds of people in a Sunni district of Latakia, causing scores of injuries and possible deaths, residents said.

One witness said he saw water trucks hosing down blood on the streets near the Takhasussieh School in the Sleibeh district of Latakia, Syria's main port, 330 km  northwest of the capital Damascus.

One cannot move two steps in the streets without risking arrest. It is difficult to know if there were deaths, but we heard heavy AK-47 fire,' a resident said.

'One thing is certain. This regime of thugs is exposing its fangs. Brutality is the only thing it knows,' he said by telephone.

European Union foreign policy chief Catherine Ashton strongly condemned the violence and deaths, and urged Syria to implement 'meaningful political reforms'.

There were rallies on Friday from the Mediterranean port of Latakia to Albu Kamal on the Iraqi border, as demonstrations entered a fourth week in defiance of Assad's security crackdown and despite his growing list of reform pledges.

The rights group said in a statement that 30 people were killed on Friday in Deraa, the epicentre of protests. It added that three were killed in the central city of Homs and three others in Harasta, a Damascus suburb, as well as one in Douma.

Latakia, where Saturday's early shooting took place, is a majority Sunni city, with significant numbers of Alawites, who follow an offshoot sect of Shi'ite Islam, and Christians.-Reuters