BEIRUT — Syrian troops opened fire Friday during an anti-government demonstration that appeared to be one of the biggest yet, reportedly killing dozens of protesters and wounding many more in the central town of Hama as Syrians around the country poured onto the streets in a renewed effort to force the ouster of President Bashar al-Assad. According to eyewitness reports and human rights groups, tens of thousands of people had gathered in a central square in Hama, 116 miles north of Damascus, when security forces began shooting randomly into the crowd.