Some 200 members of Syria's ruling Baath party are reported to have resigned over the violent crackdown against pro-democracy demonstrations.
The resignations were centred on the southern city of Deraa, a focal point of violence that has allegedly killed 450 people in six weeks.
Shooting was heard in Deraa overnight, where the government this week sent tanks and troops to regain control.
Meanwhile, the UN failed to agree on a statement condemning the crackdown.
A draft proposed by France, Britain, Germany and Portugal was opposed by several states within the 15-member Security Council, with Russia insisting events in Syria were not a threat to international peace.
President Bashar al-Assad's government disputes the Western view that the demonstrations have been non-violent.
In a statement carried by the official news agency, it said it had sent troops to several cities on the request of citizens who were worried about "armed extremists". Opposition leaders say the protests are peaceful.
In the UK, meanwhile, the invitation of the Syrian Ambassador Sami Khiyami to Friday's royal wedding has been withdrawn, after Dr Khiyami was summoned to the Foreign Office and told that the violence in Syria was "completely unacceptable".
Foreign media ban
A declaration signed by the Baath party officials in Deraa said they were quitting to protest against the violent crackdown, Reuters reported.
"In view of the negative stance taken by the leadership of the Arab Socialist Baath Party towards the events in Syria and in Deraa, and after the death of hundreds and the wounding of thousands at the hands of the various security forces, we submit our collective resignation," said the statement.
The resignations follow those of 30 Baath officials from the restive coastal city of Baniyas, north-west of Damascus.
On Thursday, Lebanese security officials said several hundred Syrians, mainly women and children, had fled on foot across the border into a remote area of northern Lebanon, after disturbances broke out in the nearby Syrian town of Tel Kalakh. There were reports of shooting and demonstrations in other towns north of Damascus.
Meanwhile residents of Deraa told the BBC the city is in "complete lock-down", with no-one being allowed to leave their homes.
One resident said the streets were filled with tanks and snipers, and people were afraid of going outside for fear of being shot, meaning that plans for a protest on Friday have been cancelled.
On Wednesday, footage posted on the internet appeared to show Syrian tanks heading towards Deraa to reinforce troops who had moved in on Monday.
Amnesty International quoted eyewitnesses as saying army snipers were shooting at wounded residents lying in the streets and that other people were trying to save them.
But with communications apparently cut off from the city, it is hard to be sure what is happening there.
Despite the crackdown, though, protesters have designated Friday a "day of rage" throughout the country. They say there is no turning back.
'No need to investigate'
On Friday, the UN Human Rights Council is due to hold an emergency session to consider a draft resolution demanding an immediate end to President Assad's efforts to crush the challenge to his rule.
The text also calls on Syria to lift its ban on nearly all foreign media and ease its restrictions on the internet and telecommunications.
At the UN on Wednesday night, China and India called for political dialogue and peaceful resolution of the crisis, but stopped short of condemning the violence.
Alexander Pankin, Russia's Ambassador to the UN, warned that a "real threat to regional security could arise from outside interference in Syria's domestic situation".
Moscow has increasingly opposed military action in Libya, arguing that operations against Col Gaddafi's forces have exceeded the scope of a Security Council resolution.
But on Thursday its foreign ministry called on Damascus to "bring to justice" those responsible for deaths at protests across the country, Tass news agency reported.
Syria's ambassador to the UN, Bashar Jaafari, said his government would resist external intervention in his country's affairs.
"As a government, we cannot accept that some claim to value the lives of our sons more than we do. The policies of interfering in affairs of other states through various justifications and pretexts have always proven to be erroneous," he said.