DARAA, Syria -- Syrian police launched an 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 on 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.