Clashes have been reported near the Syrian government headquarters after rebels attacked forces loyal to President Bashar al Assad - a day after three of his top aides were killed in a bomb blast.
At least one person was believed to have died in the latest fighting in the Ikhlas neighbourhood next to the Council of Ministers, a huge complex, and a Damascus University campus, activists and residents said.
Mr Assad's forces have deployed armoured vehicles and increased roadblocks across the capital.
More than 200 people were thought to have died in a day of violence in the country on Wednesday.
It comes as the president is in the coastal city of Latakia, directing how his regime responds to the bomb attack, according to opposition sources and a Western diplomat.
The high-profile blast, which targeted a meeting of Mr Assad's security chiefs in the capital Damascus, killed his brother-in-law Assef Shawkat, defence minister General Daoud Rajha and General Hassan Turkmani, head of the regime's crisis team on the 16-month uprising.
Among the wounded were interior minister Mohammed al Shaar and General Hisham Ikhtiyar, head of national security.
Latakia is the ancestral home of the ruling Alawites and near the city there is a palace, which Mr Assad has used before to conduct official business.
Mr Assad has not made a public appearance since Wednesday's bombing.
If he is in the Mediterranean sea resort, which has a port, it is not clear if he went there before or after the attack.
"Our information is that he is at his palace in Latakia and that he may have been there for days," said a senior opposition figure, who declined to be named.
Meanwhile, Prime Minister David Cameron says the president must step down following the escalation of violence.
Mr Cameron also appealed to Russia and China to support a new UN resolution that threatens non-military sanctions and is tied to Chapter 7 of the UN Charter, which could eventually allow the use of force to end the conflict.
He made the call in Kabul, where he is meeting the Afghan president and Pakistani prime minister to discuss the ongoing war in Afghanistan.
The British leader says it is time for Mr Assad "to go", saying that otherwise civil war is inevitable.
Violence has engulfed the capital since the rebel Free Syrian Army announced on Monday the launch of Operation Damascus Volcano "in response to massacres and barbaric crimes" committed by the president's regime.
One Syrian security official said the bombing that killed three top Assad aides was carried out by a bodyguard of one of the ministers or security chiefs at the meeting and the attacker had been wearing an explosives belt.
Another official said the blast was caused by a briefcase packed with explosives that a bodyguard left in the meeting room and detonated from a distance via remote control.
State media initially said it was a "suicide bombing" before apparently retracting and calling it a "terrorist attack".
