Syrian regime forces have fired on protesters who took to the streets of Aleppo, wounding several people at the biggest rally seen in the country'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," Syrian Observatory for Human Rights spokesman Rami Abdel Rahman said.
"These are the most important events in Aleppo since the beginning of the revolt."
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," he said.
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, north-eastern Hasaka, Homs in central Syria, and north-western 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 on 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.
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.