DARAA, Syria (AP) — Syrian police launched a relentless assault Wednesday on a neighborhood sheltering anti-government protesters, fatally shooting at least 15 in an operation that began before dawn, witnesses said.

At least six were killed in the early morning attack on the al-Omari mosque in the southern agricultural city of Daraa, where protesters have taken to the streets in calls for reforms and political freedoms, witnesses said. An activist in contact with people in Daraa said police shot another three people protesting in its Roman-era city center after dusk. Six more bodies were found later in the day, the activist said.

Inspired by the wave of pro-democracy protests around the region, the uprising in Daraa and at least four nearby villages has become the biggest domestic challenge since the 1970s to the Syrian government, one of the most repressive in the Middle East. Security forces have responded with water cannon, tear gas, rubber bullets and live ammunition. The total death toll now stands at 22.

As the casualties mounted, people from the nearby villages of Inkhil, Jasim, Khirbet Ghazaleh and al-Harrah tried to march on Daraa Wednesday night but security forces opened fire as they approached, the activist said. It was not immediately clear if there were more deaths or injuries.

Democracy activists used social-networking sites to call for massive demonstrations across the country on Friday, a day they dubbed “Dignity Friday.”

US State Department spokesman Mark Toner said Washington was alarmed by the violence and “deeply concerned by the Syrian government’s use of violence, intimidation and arbitrary arrests to hinder the ability of its people to freely exercise their universal rights.”

Heavy shooting rattled Daraa throughout the day, and an Associated Press reporter in the city heard bursts of semi-automatic gunfire echoing in its old center in the early afternoon.

The London-based Syrian Human Rights Committee reported on its website, quoting sources in Daraa, that Syrian authorities shot and killed soldier Khaled al-Masri for refusing orders to take part in storming al-Omari mosque. The report could not be independently confirmed.

State TV said that an “armed gang” had attacked an ambulance in the city and security forces killed four attackers and wounded others and was chasing others who fled. It denied that security forces had stormed the mosque, but also showed footage of guns, AK-47s, hand grenades, ammunition and money that it claimed had been seized from inside.

A video posted on Facebook by activists showed what it said was an empty street near al-Omari Mosque, with the rattle of shooting in the background as a voice shouts: “My brother, does anyone kill his people? You are our brothers.” The authenticity of the footage could not be independently verified.

Mobile phone connections to Daraa were cut and checkpoints throughout the city were manned by soldiers in camouflage uniforms and plainclothes security agents with rifles. Anti-terrorism police wearing dark blue uniforms were also out on the streets.

An ambulance was parked on the side of a road leading to the old city, its windshield smashed.

The witness said hundreds of anti-terrorism police had surrounded al-Omari mosque.

The activist in Damascus said six had been killed in the raid on the mosque, which began after midnight and lasted for about three hours. A witness in Daraa told the AP that five people had been slain, including a woman who looked out her window to see what was happening during the operation.

The activist said witnesses saw the body of a 12-year-old girl near the mosque late Wednesday afternoon. Another man was fatally shot by police after a funeral for one of the slain, the activist said.

And four more bodies were seen laying near the offices of a security agency but no one dared to come and pick them up, the activist said.