Large rallies urging further defections from the Syrian security forces have been attacked by government gunmen, as a senior United Nations official warned that the country was sliding towards a full-blown civil war.

At least 10 people were killed across Syria, taking the death toll in the six-month uprising to more than 3,000, not including loyalist members of the security forces who continue to target demonstrators.

Deir Azour in the Kurdish north was a scene of violent clashes between security forces and defectors who had been urged to swap sides by thousands of chanting activists.

In recent weeks a largely passive rights movement inside Syria has taken on a more militant tone, with activists openly seeking weapons and soldiers who have fled the army battling with their former colleagues. The Syrian government says around 1,100 security officers have been killed.

On Thursday, at least 36 people were killed nationwide, 25 of them either former or serving security force members. Friday's demonstrations were called in support of the defectors, who Syrian activists say hold the key to the future of an uprising that has become a grinding struggle.

"We know the world is not coming to help us," said one man speaking by phone from Homs. "We will do what we have to do. Our brothers in the security forces are increasingly waking up. And soon we will fight alongside them in large numbers."

The UN human rights commissioner, Navi Pillay, called for more to be done to stop the violence. "The onus is on all members of the international community to take protective action in a collective manner, before the continual ruthless repression and killings drive the country into a full-blown civil war," she said in a statement.

"As more members of the military refuse to attack civilians and change sides, the crisis is already showing worrying signs of descending into an armed struggle."

The UN and the international community have been accused of being flat-footed in their response to the crisis, in stark contrast to the enthusiastic intervention in Libya that led to Muammar Gaddafi's ousting in August.

The US and Europe have ruled out military intervention and Washington's push for more aggressive sanctions against Damascus was stymied last week by the UN security council after Russian and Chinese vetoes.

The US and Britain this week demanded that the Syrian regime stop intimidating activists abroad. US officials announced the arrest of a Syrian national who they claimed had filmed and threatened activists in Washington and then flown to Damascus to discuss opposition activities inside the US with the president, Bashar al-Assad, and his inner sanctum.

Britain called in the Syrian ambassador to warn against using embassy officials to threaten UK-based activists. Amnesty International recently claimed regime monitoring of activists was taking place in eight countries.

In Lebanon, the Syrian ambassador Ali Abdul Karim Ali denied claims that Syrian officials had been responsible for the abduction of three opposition activists near Beirut.

The Lebanese security force chief, Major General Ashraf Rifi, said the alleged evidence against the embassy was "conclusive". Sources close to him said they had video evidence, written testimonies and intelligence evidence that proved the embassy was active in the abductions. Ali called on him to publicly reveal what he had.

Assad this week claimed that the "most difficult phase" of the uprising had passed. His supporters staged a mass rally inside Damascus, which was cast almost as a victory parade.

However, Friday's protests erupted in areas that the regime was considered to have controlled in recent months. The city of Homs remains out of its control, with armed demonstrators protecting neighbourhoods and the Syrian military stationed on the city's perimeter.

The former Lebanese president Amin Gemayel said Assad had little option but to continue with the lethal crackdown if he intends to try to cling to power. "Such a regime needs a minimum of brutal repression. Without it he won't be able to lead the country," he said.

"His regime has been built on fear and repression and if you take that away he has no legitimacy. If the people start to lose their fear he is finished. But they are not there yet."

A protester from Homs said the city feared no one. "Tell Bashar he is finished and we will prevail," he said. He would not give his name.

• This article was amended on 17 October 2011. The original said Amnesty International had claimed the Syrian regime's monitoring of activists was taking place in around 30 capitals. Amnesty found about 30 cases in eight countries. This has been corrected.