'Thousands' protest in northern Syrian city of Aleppo
Thousands of people have taken to the streets in the northern Syrian city of Aleppo, activists say, in the biggest protests there since the revolt began.
The Syrian Observatory for Human Rights said several people were injured by tear gas and live ammunition.
Protests were reported elsewhere, a day after activists called for rallies in solidarity with students in Aleppo.
Observers say anti-government sentiment is rising there, after security forces killed students in a raid in early May.
Videos posted by activists show hundreds of people taking part in anti-government rallies in various parts of Aleppo.
Syria's second city has so far not experienced the violence seen in other cities during the uprising and has remained largely loyal to the government of President Bashar al-Assad since protests began in March 2011.
Analysts say its ability to keep control of Aleppo is one of its key tests.
Separately, the UN Secretary General Ban Ki-moon has said that Islamist militants from al-Qaeda must be behind two deadly suicide car bomb attacks in the capital on 10 May.
Mr Ban also said the death toll in the Syrian crisis now stands at 10,000 people.'University of revolution'
Syrian activists reported protests on Friday in Damascus, Homs and Idlib.
State TV said government forces had foiled an attempt to blow up a booby-trapped car carrying approximately 600 kg of explosives in the eastern province of Deir Ezzor on Friday.
The Friday protests in Aleppo came a day after students held a large rally at the city's university.
Activists streamed their rally live on the internet on Thursday. They were demanding the removal of Mr Assad and his government during a visit by UN observers.
Footage showed scores of demonstrators chanting loudly. A YouTube clip apparently from the same rally shows students climbing on top of a UN vehicle.
A voice could be heard in the background saying in English "the university of the revolution".
Another YouTube video, filmed through the windscreen of a car, appears to show a demonstrator being beaten by a group of men wielding sticks.'Dampening effect'
Nearly 260 unarmed UN observers have been deployed in Syria to monitor a ceasefire, which has largely collapsed since it was brokered in March by UN and Arab League representative Kofi Annan.
The UN secretary general said their deployment had some "dampening effect" on the violence, but not enough to halt it.
"We are trying our best efforts to protect the civilian population," Mr Ban added.
Last week's suicide car bomb attacks in Damascus killed 55 people and injured 372 - the deadliest attack on the city since the uprising against President Assad began.
Syrian officials have blamed "foreign terrorists" for the twin bombings.
Earlier this month Syria sent a list of 26 names to the United Nations of foreign nationals it had apprehended, claiming most of them were members of al-Qaeda.