BEIRUT — Gunfire and explosions erupted Thursday in the city at the heart of Syria's anti-government uprising as soldiers launched a massive crackdown, witnesses said. Terrified residents cowered inside their homes and used mosque loudspeakers to call for blood donations to help the wounded.

Details about the siege in Homs were sketchy, as most witnesses told The Associated Press they were too scared even to look out their windows. The city has seen some of the most intense violence as the regime tries to stamp out a revolt that has lasted more than four months.

"I can see smoke billowing from the neighborhood," a witness told The Associated Press by telephone from the Bab Sbaa area of Homs, about 100 miles from Damascus. Heavy gunfire crackled in the background. "We cannot leave our homes."

Calls for blood donations blared from mosque loudspeakers, raising fears of mass casualties. But the gunfire was too intense for people to collect any victims.

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As darkness fell, another resident said the violence had tapered off, with only intermittent cracks of gunfire. He said Syrian soldiers in personnel carriers were leaving the area.

Both spoke on condition of anonymity for fear of reprisals by the government.

The regime has banned nearly all foreign media and restricted coverage, making it nearly impossible to independently verify events on the ground or casualty figures.

Activists say up to 50 people have been killed in Homs since Saturday, a wave of violence that has signaled a potentially dangerous turn in the uprising.

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