An advance party of six United Nations observers will begin monitoring the Syrian ceasefire today.
24 others are expected to arrive in the coming days.
Monitors say Syrian forces have killed two civilians and are locked in fierce clashes with rebels in one city while bombarding opposition bastions in another, hours after UN observers arrived.
Syria's ceasefire is four days old.
Five civilians were killed yesterday in shelling of the city of Homs - UN-Arab League envoy Kofi Annan described the ceasefire as being "very precarious".
Even though the overall level of violence has dropped, escalating regime attacks over the weekend raised new doubts about President Bashar Assad's commitment to a plan by special envoy Kofi Annan to end 13 months of violence and launch talks on Syria's political future.
Mr Assad has halted shelling of rebel-held neighbourhoods, with the exception of Homs, but ignored calls to pull troops out of urban centres, apparently for fear of losing control over a country his family has ruled for four decades.
Rebel fighters have also kept up attacks, including shooting ambushes.
Reports from Syria cannot be independently verified as state authorities have barred international journalists and rights groups.
The international community hopes UN observers will be able to stabilize the cease-fire, which formally took effect Thursday.
A six-member advance team of UN observers headed to Damascus on Sunday, a day after an unanimous UN Security Council approved such a mission.
A larger team of 250 observers requires more negotiations between the U.N. and the Syrian government next week.
UN Secretary-General Ban Ki-moon expressed serious concern at the Syrian government's shelling of Homs and said "the whole world is watching with sceptical eyes" whether the cease-fire can be sustained.
"It is important - absolutely important that the Syrian government should take all the measures to keep this cessation of violence," he told reporters in Brussels after meeting Belgian Prime Minister Elio Di Rupo on Sunday. "I urge again in the strongest possible terms that this cessation of violence must be kept."
Mr Ban said he had in-depth discussions Saturday with Annan in Geneva and expressed hope that once the full monitoring team is on the ground "there will be calm and stability and peace without any violence."
42 protesters holding candle-lit vigil arrested
A Syrian rights activist expressed alarm on Monday after 42 protesters among 500 people holding a candle-lit vigil last week outside parliament demanding an end to killings were arrested, some violently.
A group of youths carrying candles staged the sit-in on Thursday in front of the parliament building in Damascus, "demanding an end to the killings," said Anwar Bunni, who heads the Centre for Legal Research and Studies.
"42 people were arrested, and some of them were seriously hurt" Bunni wrote on his Facebook page, denouncing the "savagery employed against peaceful protesters."
"They carried candles... to demand an end to the killings. They were attacked with massive brutality by security forces, who hit the demonstrators, including aged women," he wrote.
Although protests against the regime of President Bashar al-Assad have raged in Damascus suburbs and the provinces since March last year, they remain rare in the heart of the capital.
Last week, a woman was arrested after staging a solo protest outside parliament, also to demand an end to the killings which monitors number at more than 10,000 people since March last year.
A YouTube video posted online shows the young woman holding a red placard bearing the message: "Stop the killings. We want to build a Syria for all Syrians."
On Saturday, security forces also arrested journalist and writer Marie Issa and her husband, urologist Joseph Nakhle, "in front of their children," at their home in the Damascus suburb of Jaramana, said the Syrian Observatory for Human Rights.
It said they were arrested for their "peaceful activities in support of the revolution in Syria."
