Tens of thousands of protesters defied regime gunfire and took to the streets on Friday, a day after twin bombings killed dozens of people in Damascus, a monitoring group said.
Troops shot and wounded five protesters in the capital and 20 in the Hama town of Helfaya, where two civilians also died, and they killed one demonstrator in the northern city of Aleppo, the Syrian Observatory for Human Rights said.
Aleppo-based activist Mohammed al-Halabi said the protester died from his wounds after regime forces opened fire in the Salaheddine neighbourhood.
For its part, state television said troops killed a would-be suicide bomber in the city.
"The Syrian authorities have foiled an attempted suicide attack in Al-Shaar area in Aleppo, and killed the would-be attacker," the channel added, saying the attacker's car was laden with 1,200 kilos (2,640 pounds) of explosives.
Halabi said "thousands of people are protesting in spite of gunfire. They are condemning the criminality of yesterday's bombing."
They also condemned the United Nations for failing to stop the violence in Syria, calling for "immediate international military intervention," Halabi said.
The crackdown took place a day after Thursday's bomb attacks struck the capital during the morning rush hour.
They were the deadliest in 14 months of unrest, killing 55 people and wounding nearly 400, to a chorus of international condemnation.
The United Nations called on both sides in the conflict to cooperate with a month-old ceasefire as President Bashar al-Assad's regime and the opposition traded accusations over the perpetrators of the Damascus carnage.
Elsewhere in the country, five civilians were wounded when regime troops opened fire in the Tadamon neighbourhood" of Damascus to quell protests, said the Britain-based Observatory.
Thousands of people took part in anti-regime demonstrations across Syria after the weekly Muslim prayers, with the Observatory singling out Idlib province in the northwest, Hama in the centre and the eastern province of Deir Ezzor.
Thursday's bombings have raised fears that extremist elements could be taking advantage of the deadlock in Syria to stoke the unrest.
World powers condemned the attacks that targeted a military intelligence building and urged both sides to the conflict to adhere to the ceasefire brokered by UN-Arab League envoy Kofi Annan.
The Security Council urged the regime and rebels to "immediately and comprehensively" implement Annan's six-point peace plan, "in particular to cease all armed violence".
UN leader Ban Ki-moon, who warned earlier this week of possible civil war if Annan's plan failed, also renewed a call for all sides to cease violence and "to distance themselves from indiscriminate bombings and other terrorist acts."
Syria's UN envoy, meanwhile, said Britons, French and Belgians were among foreign fighters killed in the country's escalating conflict and that there was Al-Qaeda involvement.
In northwest Idlib, a flashpoint of unrest, protesters held up slogans reading: "When are you going to understand? There is no Al-Qaeda here," according to amateur videos posted by activists on YouTube.
Ambassador Bashar Jaafari told the Security Council 12 foreign fighters had been killed and 26 detained in recent clashes with Syrian forces, "including one French citizen, one British citizen, one Belgian citizen."
A list of the 26 detained had been sent to Ban and the Security Council.
Syria's main regional ally Iran accused Western powers of orchestrating Thursday's bombings, with First Vice President Mohammad Reza Rahimi saying the attacks were aimed at halting reforms in the country.
"The terrorist acts were guided by (global) arrogance and the enemies of free nations," Rahimi said, quoted by state news agency IRNA, using the Islamic Republic's term for Western powers.
Assad's regime and the opposition traded accusations over the attacks.
The government blamed foreign-backed "terrorists," while the main opposition coalition, the Syrian National Council, said authorities were resorting to "terrorism" to bury the Annan plan.
Middle East expert Joshua Landis wrote on his blog that, given that law and order was breaking down in Syria, "we should expect the spread of radical groups."
"The Syrian state, being one of the most intrusive and repressive in the Middle East, was able to thwart radical groups," he wrote. "As its capabilities decline, so will its ability to keep such groups from penetrating Syrian society."
The uprising in Syria began as a peaceful popular revolt but has turned into an insurgency amid mounting calls to arm rebels seeking to overthrow Assad.
More than 12,000 people, the majority of them civilians, have died since the uprising began, according to the Observatory, including more than 900 killed since the April 12 truce went into effect.
Neeraj Singh, spokesman for the UN observer mission overseeing the putative truce, said 105 monitors had so far arrived in Syria out of an expected total of 300, and had been deployed in flashpoints including central Homs and Idlib.
"Where we have our military observers on the ground, they have had a calming effect on the situation," Singh said. "At the same time, we have seen a worrying trend of improvised explosive devices being used."
In other developments, the European Union is set to slap new sanctions on Syria, imposing an assets freeze and visa ban on two firms and three people, EU diplomats said in Brussels.