As thousands protested across Syria against the government, activists say Syrian security forces killed at least 19 people on Thursday, including some in an area visited by Arab League monitors.

The Syrian Observatory for Human Rights says at least four of the deaths were in the Damascus suburb of Douma. It says security forces opened fire after thousands of people began protesting outside a mosque. Some residents said Arab League observers were in the area at the time.

Earlier, the observers announced plans to visit the Daraa, Hama and Idlib regions to check on government pledges to halt the deadly crackdown and release political detainees. The monitors also said they will make unannounced visits.

On Wednesday, activists and rights groups said government forces had killed at least 39 people since the monitors arrived Monday, including six people shot in Hama.

Some residents and activists raised concerns that the Arab observers were neither properly qualified nor independent. Residents of the hard-hit district of Baba Amr in Homs refused to allow observers in because army officers were with them. The standoff ended when the officers withdrew.

Syrian authorities agreed to the Arab League observers under international pressure and threats of Arab sanctions. The plan requires Syrian authorities to give the monitors freedom of movement except for sensitive military sites.

The United Nations estimates 5,000 people have been killed since March in violence linked to Syria's unrest. Syria says armed terrorists are driving the revolt. It accuses them of killing 2,000 security personnel since March.