Syrian opposition figures have said their "massive grassroots revolution" will break the regime unless Bashar al-Assad, the president, leads a transition to democracy.
The statement on Wednesday from an umbrella group of opposition activists in Syria and abroad, called the National Initiative for Change, said a democratic transition will "safeguard the nation from falling into a period of violence, chaos and civil war."
"If the Syrian president does not wish to be recorded in history as a leader of this transition period, there is no alternative left for Syrians except to move forward along the same path as did the Tunisians, Egyptians
and Libyans before them," the statement said.
The opposition in Syria is getting more organised as anti-government protests gain strength, but it is still fragmented.
Meanwhile, witnesses said a convoy of about 30 tanks were seen on the Damascus circular highway outside the capital.
Witnesses also said troops had poured into the Damascus suburb of Douma overnight and were also deployed around the coastal city of Baniyas.
White buses brought in hundreds of soldiers in full combat gear into Douma, a witness told Reuters news agency. Pro-democracy protesters have tried to march from the suburb into the centre of the capital in the last two weeks but have been dispersed by security forces.
More than 2,000 security police deployed in Douma on Tuesday, manning checkpoints and checking identity cards to arrest pro-democracy sympathisers, the witness, a former soldier, said.
He said he saw several lorries in the streets equipped with heavy machine guns and members of the plainclothes secret police carrying assault rifles. He believed the soldiers to be Republican Guards, among the units most loyal to Bashar al-Assad, the president.
Troops have been deployed in the southern city of Deraa since Monday and activists said gunfire could be heard on Wednesday.
Sawasiah, a Syrian human rights organisation, said security forces had killed 35 civilians since they entered the city. The group said electricity, water and telecommunications were cut in Deraa and that supplies of blood at hospitals was starting to run low.
Late on Tuesday, the state news agency SANA reported the army "continued to chase armed groups and extremists in Deraa who attacked military positions, cut off roads and forced passers-by to stop so they could hit them."
The Syrian Observatory for Human Rights said on Wednesday it had collected the names of at least 453 civilians killed during almost six weeks of pro-democracy protests.
Asked who killed them, the group's director Rami Abdelrahman said: "It does not require a comment. The names we have are from Deraa, Damascus, rural Damascus and the coast."
Meanwhile, the official news agency SANA said funerals were to be held for six security personnel killed on Tuesday by "armed extremist groups".
The UN's top human rights body has agreed to hold a special human rights session on Syria to try to stop the security forces' brutal crackdown on protesters.
The 47-nation Human Rights Council in Geneva will convene the special session on Friday. The meeting was requested by the US and endorsed by 16 member states including Britain, France, and Japan.
No Arab countries were among those requesting the session, which requires endorsement by one-third of the forum's membership to convene. Arab diplomats to the UN were holding closed-door talks on the issue in Geneva, according to UN sources.
Emergency sessions in recent months have launched investigations into alleged human rights violations in Libya and Ivory Coast.
The UN secretary-general has called for an independent inquiry into the deaths of people he has described as peaceful demonstrators.
But Syria's UN envoy has said the country is perfectly capable of conducting its own transparent inquiry into the deaths.
Ambassador Bashar Ja'afari told reporters that Assad had instructed the government "to establish a national commission of inquiry and investigation about all the casualties among civilians" and the envoy pledged "full transparency".
"We have nothing to hide," he said.
"We regret what's going on, but you should also acknowledge the fact that this unrest and riots, in some of their aspects, have hidden agendas," he said, adding that some foreign governments were trying to destabilise Syria.
Meanwhile International pressure on Assad is mounting, with European governments urging Syria to end the violence.
France, Britain, Germany, Italy and Spain told Syrian ambassadors in a co-ordinated effort that they condemned the recent crackdown and that Assad must change his ways, France's foreign ministry said.
The ministry said France expressed its "firm condemnation of the escalation of the repression by Syrian authorities against the population" and called on Syria to respect its international obligations on human rights.
European Union governments will discuss the possibility of imposing sanctions against Syria on Friday, with various measures being explored, a spokesman for EU foreign policy chief Catherine Ashton said.
"All options are on the table," he said.