Omar Hamza, a witness to the violence Friday in the Damascus suburb of Douma, said government forces shot at protesters who gathered at a mosque in the city, al-Jazeera reported.
"More than 100 people are injured right now," Hamza said. "It is a very bad situation in Douma today."
Opposition activists called for people to rally to show Arab League monitors in Syria the widespread anti-government sentiment.
"The observers saw a lot of violence in the city. They saw how security forces shoot at protests. They also saw the bodies of dead people," Hadi Abdullah, an activist in Homs, told al-Jazeera.
Arab League monitors are to determine whether the government of Syrian President Bashar Assad was honoring a pledge to end its violent crackdown on pro-democracy demonstrators, the BBC reported.
The Arab League peace plan calls for an end to the violence, withdrawal of armed forces and the release of all detainees.
One activist in Douma said protesters gathered early Thursday to greet what they thought was a busload of observers but saw security forces instead, The New York Times reported.
"We lost six people, the price of seeing this cursed mission," the activist said. "Their presence has raised the killing, in fact."
The mission has drawn criticism from activists, including Syrian dissident Haytham Manna, who called for the delegation's leader to be replaced or have his powers reduced. Activists have complained that Sudanese Lt. Gen. Muhammed al-Dabi is unsuitable because he has been accused of committing atrocities in Sudan.
Reports of violence prompted Britain's Foreign Office Friday to say it welcomed news that the Arab League planned to increase the number of observers in Syria, CNN reported. An Arab League official said 75 monitors were in Syria and more were expected to arrive in the coming days.
"Unfortunately, reports show that the violence has continued in Syria over the past few days," Alistair Burt, the Foreign Office's minister for the Middle East and North Africa, said in a statement.
"I urge the Syrian government to meet fully its obligations to the Arab League, including immediately ending the repression and withdrawing security forces from cities. The Syrian government must allow the Arab League mission independent and unrestricted access to all areas of Syria and to the Syrian people."