A heavy firefight has broken out between Free Syrian Army rebels and forces loyal to President Bashar al-Assad in a main district of the capital Damascus, Reuters news agency reported.
The sound of heavy machineguns and rocket-propelled grenades on Monday echoed throughout the night from the western neighbourhood of al-Mezze, one of the most heavily guarded areas of the capital and home to several security installations, residents told the agency by telephone.
The latest fighting came a day after a car bomb in Syria's second biggest city of Aleppo killed at least three people, according to the Syrian Observatory for Human Rights has said.
Residents told the UK-based rights group that they saw bodies in the streets after Sunday’s blast close to a state security office.
State news channel Syria TV said that Sunday's "terrorist" explosion took place between two residential buildings in the al-Suleimaniya district, behind a post office.
Al Jazeera's Zeina Khodr, reporting from neighbouring Lebanon, said it is not the first time Aleppo was hit during the now year-long uprising against the rule of President Bashar al-Assad.
"We have seen an explosion in Aleppo just a few weeks ago. That explosion targeted the security headquarters."
The opposition also reported heavy raids by security forces and fighting with rebels in northern and southern Syrian provinces and suburbs of Damascus.
In the capital, as crowds gathered for memorials to the 27 victims of Saturday's car bombs, security forces broke up an opposition march of more than 200 people when protesters began shouting "the people want to topple the regime".
The phrase has echoed through the wave of Arab uprisings that began last year and has toppled autocratic rulers in Tunisia, Egypt, Libya and Yemen.
"They were walking through an area in central Damascus, near SANA (the state news agency),” Rami Abdelrahman from the Syrian Observatory for Human Rights said.
"At first they shouted slogans against violence and the police didn't do anything, but as soon as they started to call for regime change the police rushed in and started beating people with canes," Abdelrahman said.
The protest, which called for non-violent resistance to the government, had been led by moderate opposition leaders previously tolerated by the government because of their calls for dialogue and rejection of foreign intervention.
State television blamed "terrorists" for the Saturday morning explosions and reported that vehicles packed with explosives had been used.
The blasts targeted buildings belonging to a customs office and air force intelligence. Most of the casualties were civilians, state television said.
| Al Jazeera's Zeina Khodr reports from Lebanon where Islamist groups support the Syrian opposition
The channel broadcast interviews with Syrians who blamed the attack on the United States and Gulf nations, including Saudi Arabia and Qatar, who they said had sent "terrorists".
Bassma Kodmani, a Paris-based member of the opposition Syrian National Council, said she doubted the armed groups trying to bring Assad down by force have the capacity to carry out such attacks on security institutions in the capital.
"I don't think any of the opposition forces or the Free Syrian Army has the capacity to do such an operation to target these buildings because they are fortresses," she said.
"They are very well guarded. There is no way anyone can penetrate them without having strong support and complicity from inside the security apparatus."
SANA said a third blast went off near a military bus at the Palestinian refugee camp Yarmouk in Damascus later in the day, killing the two suicide bombers.
Activists said the Sunday march in Damascus aimed to commemorate the peaceful roots of Syria's uprising, which has been overshadowed by the growing armed insurgency against state security forces.
Heavy fighting also raged in the northwestern province of Deir Ezzor and military vehicles were torched, activists said.
Rebels also blew up a bridge in southern Deraa, birthplace of the uprising, the UK-based rights group said. The bridge had been used to transport supplies to security forces besieging the city.
The rights group said security forces raided the town of Artouz, a Damascus suburb, looking for wanted men. The Local Co-ordination Committee, another opposition group, said residents they could hear heavy gunfire.
Technical experts from the UN and the Organisation of Islamic Co-operation are in Syria on a mission to assess the humanitarian impact of the bloodshed, a senior OIC official said on Sunday.
"The joint OIC-UN mission entered Syria on Friday to carry out an evaluation of humanitarian aid," on a mission led by the Syrian government, its assistant secretary general, Atta al-Mannan Bakhit, told AFP news agency.
He said the mission, with three OIC experts in the team, would cover 15 cities, after which a report would be submitted to the Saudi-based Islamic grouping and the UN on the humanitarian needs of the Syrian population.
The United Nations estimates that more than 8,000 people have died so far in the violence following the revolt against four decades of rule by the Assad family.
Authorities say they are fighting insurgents who have so far killed more than 2,000 members of the security forces.