Activist Omar al-Khani says Syrian forces attacked protesters in front of the observers
Syrian security forces have reportedly clashed with protesters, leaving at least 10 dead, as tens of thousands took to the streets across the country.
Activists said several people were injured when troops opened fire to disperse a demonstration in Douma, a suburb of the capital, Damascus.
At least five were killed in Deraa and another five in Hama, they added.
The opposition called on people to join rallies to show Arab League monitors the scale of anti-government anger.
The monitors are visiting the country to verify the government's implementation of a peace initiative, which demands an end to all violence, troops pulled off the streets and political prisoners freed.
Up to 40 protesters were reportedly killed on Thursday while awaiting visits from monitors, who fanned out across the country for the first time.
The UN says more than 5,000 have been killed since the uprising against President Bashar al-Assad began in March. The government says it is fighting "terrorist gangs" and 2,000 security forces personnel have died.
Casualty figures and other information are hard to verify as most foreign media are barred from Syria.
'Nail bombs'Correspondents say the presence of the monitors has emboldened the protesters.
Following Friday prayers, there were mass rallies in several suburbs of Damascus, the restive central cities of Homs and Hama, and the southern city of Deraa, where the uprising began.
The London-based Syrian Observatory for Human Rights said as many as 250,000 people had taken to the streets in the north-western province of Idlib.
"This Friday is different from any other Friday. It is a transformative step. People are eager to reach the monitors and tell them about their suffering," Abu Hisham, an activist in Hama, told the Reuters news agency.
Activists published photos purportedly showing a mass protest in the city of HamaFootage of Homs broadcast by al-Jazeera TV showed a huge crowd dancing and shouting: "Revolution, revolution Syria, revolution of glory and freedom Syria."
But a resident of central Damascus, Ram, told the BBC that heavily-armed security forces had been deployed outside his local mosque to intimidate protesters.
"They were daring people. It's the first time that they were showing their guns showing their weapons outside," he said. "So they were telling people: 'Once you just say the word, we will shoot you.'"
In the suburb of Douma, a protest by an estimated 70,000 people was broken up by troops, who fired live rounds and tear gas, activists said.
The Syrian Observatory for Human Rights said more than 20 protesters were hurt when "nail bombs" were detonated when they approached the town hall, where Arab League monitors were believed to be based.
The Observatory also said security forces opened fire on demonstrations in Hama and Deraa, killing at least five people in each city.
Another activist group, the Local Co-ordination Committees, said the nationwide death toll on Friday was 32, with nine killed in Hama, six in Deraa, six in Idlib, and four in Tal Kalakh, near the border with Lebanon.
The bodies of five members of the same family were found in the Deir Baalbah area of Homs, a day after they were arrested, it added.
'Barrier of fear broken'The Arab League observer mission has faced criticism for being led by Sudan's Gen Mustafa al-Dabi, who Amnesty International has accused of carrying out human rights violations in his own country.
But the League says Gen Dabi has full support, and the US has urged detractors to allow the team to finish its work.
Russia's foreign ministry says the first comments from the observers showed the situation in Syria was "reassuring".
The comments came in an interview Gen Dabi gave on Thursday after a visit to Homs.
"Some places looked a bit of a mess but there was nothing frightening," he told Reuters.
But the Local Coordination Committees, a Syrian activist group that documents and organises protests, said that 130 people had been killed since the monitors arrived in the country earlier this week.
The anti-government Free Syrian Army says it has requested a meeting with the observers but received no response.
"We haven't been given any of the [telephone] numbers for the monitors, which we have requested. No one has contacted us either," said Col Riad al-Asaad, who heads a force claiming to be 10,000-strong, many of whom defected from the Syrian army during the uprising.
The rebel commander told Reuters his forces had been ordered to stop all attacks on government troops since the arrival of the observers in the country.
Rami Abdul-Rahman, head of the Syrian Observatory for Human Rights, said the Arab League's initiative was "the only ray of light" for Syrians.
"The presence of the observers in Homs broke the barrier of fear," he told AFP.
Despite the presence of the Arab monitors - who are being escorted around the country by state security officials - there has been little let-up in the ferocity of the response to protests, correspondents say.
The US State Department and UK foreign office have expressed concern about the continuing violence.
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