Syrian security forces shot dead 17 protesters on Thursday, six of them in a city being visited by Arab League monitors checking on President Bashar al-Assad's compliance with a pledge to stop a military crackdown on popular unrest.
The Arab League is hoping its deal with Assad can put an end to nine months of bloodshed in which well over 5,000 people have been killed by a United Nations count, provoking international sanctions against Damascus and raising fears of wider conflict.
But anti-Assad activists have said the monitoring mission is too small and easily restricted by state security agents determined to cover up abuses. There have also been questions about its leader, a Sudanese general whose government has defied an international war crimes tribunal over bloodshed in Darfur.
The Syrian Observatory for Human Rights said Assad's security forces opened fire on protests in cities around the country. Six died in Hama, a major hotbed of unrest, shortly after the arrival of monitors for a first look.
The Observatory said security forces fired at a street rally in Douma, a Damascus suburb, killing four people. Enraged residents launched a civil disobedience campaign and thousands reportedly flooded the main square for a sit-in.
The activists have called for complete civil disobedience. The roads have been blocked, stores are shut down and the city is paralysed, said Rami Abdelrahman, head of the British-based Observatory.
In addition to the 17 slain protesters, five unidentified people were shot dead in Idlib, near the Turkish border, in the vicinity of military sites in the city, which was also due to be toured by an Arab League delegation.
Monitors were to check conditions in the turbulent cities of Deraa, Hama and Idlib, which lie along a 450-km (280-mile) arc of revolt from the south to the north of Syria.
At its midpoint is Homs, where the Arab League mission had a controversial start when its Sudanese chief, General Mustafa al-Dabi, reported seeing nothing frightening in an initial tour.
The Arab mission is the first notable international involvement in Syria's crisis. The uprising against 41 years of repressive rule by the Assad family was inspired by other Arab popular revolts that have overthrown three dictators this year.
Anti-Assad activists said they had seen no sign of monitors on the streets of Hama and Deraa by mid-afternoon, and they were unable to contact them by telephone. Extra security forces were deployed around areas expecting monitors, they said.
Where are they? We worked very hard for this visit, we got witnesses and documented deaths and sites of shelling. People wanted to march but the monitors are missing. The security presence is really strong - it looks like they have been preparing as much as we have, said activist Odei in Idlib.
In Hama, activists said protesters went down into the streets in Hama to await the Arab League delegation, amid heavy security with snipers pointing guns out of top floor windows.
People really hope to reach (the monitors). We do not have much access to the team. The people stopped believing anything or anyone now. Only God can help us now, said Abu Hisham, an opposition activist in Hama.
Hama has a haunting resonance for Syrians opposed to Assad. His late father ordered a massacre of up to 30,000 people there in 1982 to put down an Islamist insurrection.
A source in the Arab League mission's operations centre in Cairo said earlier on Thursday there had been a problem with communications but the monitors' schedule was holding up.
We have contacted our teams ... Today's plan will not be changed and the only problem we faced today was the bad phone network, which made our communication with the monitors harder. It took more time to reach them and determine their locations.
Unless it can establish its credibility by proving it has unobstructed access to all areas and is able to hear uncensored accounts, the Arab League mission may not be able to satisfy all sides that it can make an objective assessment of the crisis.
Fifteen people were killed on the monitors' first day.
Assad says he is fighting Islamist militants steered from abroad. He says over 2,000 security force men have been killed.
In Homs, Syria's third largest city and epicentre of anti-Assad ferment, protesters said they were already fed up with the monitors who they said seemed unsympathetic and hard to find.
This mission is a big lie. They say they were in Khalidiya neighbourhood. I haven't seen them, said Tamir, shouting by telephone over protester chants of down with the regime.
We've been here at the protest. Where were they?
The head of the main opposition group in exile, The Syrian National Council, met Arab League officials in Cairo to discuss the monitoring mission.
The delegation should have been bigger because the points of confrontation and violence are much greater than the number of the monitors, and they should have better logistical means to enable them to move quicker, Burhan Ghalioun told reporters.
Some 150 monitors in all are expected to enter Syria by the end of the week. But activists say Syrian government or security officials escorting monitors can intimidate residents who want to testify about the violence.
They also say there are not enough monitors to see the full scope of unrest in a large country of 23 million people.
An international peace mission in Kosovo in the late 1990s deployed 2,000 observers in a territory of two million people one-seventeenth the size of Syria.
Ghalioun blamed Assad's government for the monitors' restrictive conditions: So far the Syrian regime has not changed its style of lies and tricks, he said.
Relentless military attacks on protests have also bred armed insurgency and thousands of rebels and army defectors have formed the Free Syrian Army (FSA) to attack military and police convoys, bases and checkpoints.
A gritty video shot by rebels in Deraa showed the ambush of a security forces convoy on Wednesday by nine gunmen who opened fire from a rooftop. The Syrian Observatory for Human Rights said four soldiers were killed in the attack by the FSA.
Syria resisted outside involvement for months but yielded to unprecedented pressure from fellow members of the 22-state Arab League last month, agreeing to let the monitors in to witness withdrawals of forces from turbulent cities.
If the Arab mission fails, the U.S. State Department said, other international means will be explored to stop the killing.
Russia and China have blocked action by the U.N. Security Council and Western powers have no appetite for military action like that which helped rebels toppled Libya's Muammar Gaddafi, given Syria's position in a web of Middle East conflicts.
(Additional reporting by Ayman Samir and Yasmine Saleh in Cairo and Suleiman al-Khalidi in Amman; Writing by Douglas Hamilton and Erika Solomon; Editing by Mark Heinrich)