Activists say some 40 people were killed by security forces on Thursday, mostly in areas where monitors are visiting, including a Damascus suburb.
The activists have called for massive street protests on Friday.
The UN says more than 5,000 civilians have died in 10 months of unrest.
The Arab League peace plan calls for a complete halt to the violence, the withdrawal of all armed forces and the release of all detainees.
However, after two days of monitoring, more questions were being asked about the head of the Arab League mission, Sudan’s Gen Mustafa al-Dabi, who Amnesty International said was responsible for “torture” and “disappearances” in 1990s Sudan.
‘Only God can help us’
After starting in the flashpoint city of Homs on Tuesday, the Arab League monitors have moved to Idlib in the north, Deraa in the south, Hama and Damascus.
Activists have reported violence and killings in all those areas.
The Syrian Observatory for Human Rights said at least four people were killed when security forces opened fire outside a mosque in Douma, a suburb of the capital.
Monitors were arriving at the city hall there when security forces fired on “tens of thousands” of protesters outside the Grand Mosque, the UK-based group said.
It reported further deaths in other suburbs of the capital – Aarbin and Kiswah – as well as in Idlib and Hama.
It also said security forces had raided a private hospital in Hama to arrest the wounded.
Casualty figures and other information are hard to verify as most foreign media are barred from Syria.
The BBC’s Jim Muir, in neighbouring Lebanon, says that far from diminishing the violence, it seems the presence of the observers may actually be causing it to increase.
One activist in Hama told Reuters: “People really hope to get to reach them. We do not have much access to the team.
“The people stopped believing anything or anyone now. Only God can help us now.”
Another, in Idlib, asked where the monitors were, saying: “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.”
Our correspondent says virtually none of the peace plan’s objectives have yet been met, although Syria on Wednesday did release 755 of the 14,000 people the UN says have been detained during the uprising.
Activists have been using social media to call for massive protests on Friday – the traditional day of demonstration.
The Syria Revolution 2011 group said: “We will march as we did in Homs and Hama where we carried olive branches only to be confronted by [President Bashar al-Assad's] gangs who struck us with artillery and machinegun fire.”
Meanwhile, more questions were being asked of Gen Dabi, after his initial comments on the mission were criticised as favouring the Syrian government.
Gen Dabi was the head of military intelligence for Sudan’s President Omar al-Bashir, who is wanted by the International Criminal Court (ICC) for his actions over Darfur.
Amnesty International said that under Gen Dabi, the military intelligence in the early 1990s “was responsible for the arbitrary arrest and detention, enforced disappearance and torture or other ill-treatment of numerous people in Sudan”.
Gen Dabi’s first comments in Syria were that he had seen “nothing frightening”, though he later said he needed more time to make an assessment.
One Arab League official in Cairo told the Associated Press Gen Dabi had the support of all its members, saying: “The mission and its final report will decide the future of Syria and this is not a small matter.”
Damascus says it has allowed the monitors “all the facilities they need”.
Foreign ministry spokesman Jihad Makdesi said this was “reflected in the positive statements made by the Sudanese chief of observers”.
President Assad says government forces are fighting armed gangs and that more than 2,000 security personnel have been killed.