|Crowds took to the streets in Homs to show support of the newly formed National Council opposition movement
At least 21 Syrians, including a prominent Kurdish rights activist, have been killed amid protests by thousands against Bashar al-Assad's government and in support of a newly formed opposition front, activists say.
Friday's surge of violence came as Russian President Dmitry Medvedev said Assad, his Syrian counterpart, would have to leave power if he failed to implement reforms acceptable to the opposition, and the Syrian government again blamed "terrorists" for the unrest.
Four civilians, including two elderly men, were shot dead in the central city of Homs by security forces and four others on the outskirts of Damascus, the UK-based Syrian Observatory for Human Rights said.
Meshaal Tammo, a 53-year-old Kurdish activist and opposition spokesman, was killed when four masked assailants stormed his house in Qamishli, in northern Syria, and opened fire, also wounding his son and another fellow activist in the Kurdish Future Party, activists said.
Thousands of Kurds took to the streets in Qamishli after Tammo's death, and gathered outside the hospital where his body was taken.
The official SANA news agency reported Tammo's "assassination", but gave a different account of his death. It said he was killed "by gunmen in a black car who fired at his car".
Tammo, a member of the newly formed Syrian National Council (SNC) opposition grouping, had been released recently after three-and-a-half years in prison.
Al Jazeera's Rula Amin, reporting from Beirut in neighbouring Lebanon, said the protests were still going strong, even after six months of demonstrations.
"The government is continuing its crackdown," she said. "The opposition insists on the fall of the regime and the government insists it will end the wave of protests."
A 10th man was shot dead on Friday by security forces in the flashpoint northern town of Jisr al-Shughur near the Turkish border, the Local Co-ordination Committees activist network reported.
In another development, Riad Seif, a prominent opposition figure and former MP, had to be given hospital treatment after being beaten outside a mosque in the capital's commercial neighbourhood of Medan.
Tammo's death and Seif's arrest were condemned by the US.
"The United States strongly rejects violence directed against peaceful oppositionists wherever it occurs, and stands in solidarity with the courageous people of Syria who deserve their universal rights," the White House said in a statement.
Protesters used Friday's demonstrations in the Damascus district of Barzeh and in Homs to show support for the newly formed SNC.
Al Jazeera's Jane Arraf reports on Syrians who have escaped the violence by fleeing to neighbouring Jordan
In the Homs neighbourhood of Qurabeyd, demonstrators raised their shoes - a sign of disrespect - alongside photographs of Assad with his face crossed out, a YouTube video showed.
Activists also documented anti-government and pro-SNC rallies in the Damascus neighbourhood of Qabun, the eastern town of Abu Kamal and Qamishli in the north, where security forces fired on protesters and arrested 10.
Security forces and paramilitaries locally known as the "shabiha" surrounded mosques in Damascus suburbs, in Homs and in the Mediterranean cities of Banias and Latakia, activists said.
In Deraa, thousands of demonstrations trampled on giant Russian and Chinese flags, in a sign of discontent at the two UN Security Council members blocking a resolution calling for "targeted measures" against Assad.
Syrian authorities have yet to comment on the formation of the SNC. But Faysal Mekdad, the country's deputy foreign minister, told the UN Human Rights Council that more than 1,100 people have been killed by "terrorists" in the revolt that has shaken the country since March.
The UN has said it now estimates the government crackdown has killed more than 2,900 people since mid-March.
Russia's Medvedev unexpectedly piled pressure on Syria, just days after Moscow and China vetoed the UN resolution.
"If the Syrian leadership is unable to undertake these reforms, it will have to go. But this is something that has to be decided not by NATO or individual European countries but by the people and leadership of Syria," ITAR-TASS news agency quoted Medvedev as saying.
In-depth coverage of escalating violence across Syria
"It's one more friend of Assad who is warning him and adding more pressure on the government in Damascus," Al Jazeera's Amin said.
"Today [Medvedev] was very blunt ... it was the starkest warning from the Russians so far to their friend in Damascus."
In wielding its veto on Tuesday, Russia said it feared the resolution could be used for military action against Syria. Russia, China and others have accused NATO of abusing UN resolutions on Libya to launch air attacks there this year.