Regime forces fired on protesters who took to the streets of Aleppo on Friday, wounding several people at the biggest rally seen in Syria's second city since a revolt erupted last year, a watchdog said.
The Syrian Observatory for Human Rights said demonstrators also suffered gunshot wounds in Douma, a key protest hub near Damascus, but did not provide any casualty figures.
"Thousands of people demonstrated in various districts (of Aleppo) despite the repression," said Rami Abdel Rahman of the Syrian Observatory for Human Rights said.
"These are the most important events in Aleppo since the beginning of the revolt," he told AFP in Beirut.
Meanwhile, the government said it had foiled a suicide bomb attack in Aleppo on May 11, a day after twin bombings in Damascus killed 55 people and wounded nearly 400. It has repeatedly blamed such attacks on "terrorists".
UN chief Ban Ki-moon said on Thursday he believed Al-Qaeda committed the Damascus attack.
"Very alarmingly and surprisingly, a few days ago, there was a huge serious massive terrorist attack. I believe that there must be Al-Qaeda behind it."
Syrian President Bashar al-Assad, as well as the United States and Russia, has already pointed to an Al-Qaeda presence in the country since the revolt against his regime began.
On Friday, Ban spokesman Martin Nesiry said there was no hard proof of that, but that the "Damascus attacks were clearly carried out by a group with organisation and intent. Some of the attacks we have seen clearly bear some of the terrorist hallmarks with which we are familiar from elsewhere."
On the ground, the Observatory said at least 11 people died in violence across the country on Friday.
Besides Aleppo, protests demanding the ouster of Assad also took place in Damascus, the eastern city of Deir Ezzor, northeastern Hasaka, Homs in central Syria, and northwestern Idlib, said the Britain-based Observatory.
The Observatory said tens of thousands of people rallied across the country, in the biggest demonstrations since an April 12 ceasefire which has been violated on a daily basis.
"We want freedom, whether you like it or not, Bashar, enemy of humanity," protesters chanted in Deir Ezzor.
The rallies came after a call by activists for Syria-wide protests under the rallying cry, "heroes of Aleppo University", in solidarity with students in the northern city who demonstrated there the day before despite brutal repression.
On Thursday, the students were met with brutal repression by security forces, despite the presence of UN military observers, who now number more than 250 across the country out of the total of 300.
One protester was killed in a separate demonstration Thursday night in the Aleppo neighbourhood of Salaheddin, according to the Observatory, while an officer was killed in a bomb explosion in the city on Friday.
Violence persisted elsewhere, with regime forces renewing their bombardment of Rastan in central Homs province on Friday, according to the Observatory, only a day after a blistering assault on the rebel stronghold.
Heavy gunfire and shelling was reported in several neighbourhoods of Homs city, said the watchdog.
Artillery attacks on towns have declined since the UN observer mission began deploying in mid-April, but the death toll is still high.
With the violence unabated, UN-Arab League envoy Kofi Annan plans to return to Damascus soon to further efforts to find a peaceful solution to the crisis, his spokesman said on Friday, without saying when.
The head of the UN observers' mission, Major General Robert Mood, told reporters in Damascus his mission "will reach full operational capabilities in record time".
But he acknowledged that, "No volume of observers can achieve a progressive drop and a permanent end to the violence if the commitment to give dialogue a chance is not genuine from all internal and external factors".
In an apparent first, Syrian authorities have sentenced to death for "treason" an activist who was arrested in April and "brutally tortured", the Syrian League of the Defence of Human Rights said.
In other developments, shelling between two pro- and anti-Assad neighbourhoods in the northern Lebanese port city of Tripoli left three people wounded, officials and hospital sources said.
Meanwhile, a top UNESCO official warned that experts had been unable to assess damage to historic treasures in Syria due to the uprising.
And Canada tightened its sanctions, targeting luxury goods believed to be coveted by Assad and his wife, as well as oil exports and Syrian airwaves.
"The ongoing, appalling violence perpetrated by the Assad regime against the people of Syria compels Canada to further isolate the regime and its backers," Canadian Foreign Affairs Minister John Baird said in a statement.
The additional measures prohibit the export of jewelry, gems, precious metals, watches, cigarettes, alcoholic beverages, perfume, designer clothing and accessories, furs, sporting goods, private aircraft, gourmet foods, lobster, computers, televisions and other electronic devices to Syria.
Under Canada's eighth round of Syria sanctions, those cited will be subject to an asset freeze and a prohibition on economic dealings.