BEIRUT, Lebanon -- Thousands of defiant Syrians chanting "We are not afraid!" were met by security forces firing bullets and tear gas Friday in a crackdown on nationwide protests that left another 42 people dead -- many of them villagers trying to break an army blockade of the southern city where the six-week uprising began.

President Bashar Assad again unleashed deadly force in a determined effort to crush the revolt, the gravest challenge to his family's 40-year ruling dynasty.

Although still in control, experts say he will struggle to recover legitimacy at home and abroad if he manages to stay in power. The United States slapped three top officials in his regime -- including his brother -- with sanctions and the UN is leading an investigation into Syria's crackdown.

Human rights groups say about 500 people have been killed since the uprising began.

Many of the 42 people killed Friday were in Daraa, said human rights activist Mustafa Osso, whose Syria-based group has been compiling casualty lists. He told The Associated Press that the death toll could rise. Thousands of people from the outskirts of Daraa tried to break the military siege on the town Friday, but security forces opened fire, witnesses and human rights groups said.

A witness in Daraa said residents stayed indoors because the city has been under siege by the military since Monday, when thousands of soldiers backed by tanks and snipers stormed in. People were too afraid even to venture out to mosques for prayers, the witness said.

A devastating picture was emerging of Daraa -- which has been without electricity, water and telephones since Monday -- as residents fled to neighboring countries. The uprising began there in mid-March, sparked by the arrests of teenagers who scrawled anti-regime graffiti on a wall.

"Nobody can move . . . [in Daraa]. They have snipers on the high roofs," a resident told the AP by satellite phone. "They are firing at everything." At the Jordanian side of the Syrian border, several Daraa residents who had just crossed said there is blood on the streets of the city.

Large demonstrations were reported Friday in the capital of Damascus, the central city of Homs, the coastal cities of Banias and Latakia, the northern cities of Raqqa and Hama, and the northeastern town of Qamishli near the Turkish border.In Damascus' central Midan neighborhood, witnesses said about 2,000 people marched and chanted, "God, Syria and freedom only!" in a heavy rain, but security forces opened fire with bullets and tear gas, scattering them.

"Oh great Syrian army! Lift the blockade on Daraa!" protesters chanted in the Damascus suburb of Barzeh, according to a video posted by activists on YouTube.

The government had warned people against holding any demonstrations Friday and placed large banners around the capital that read: "We urge the brother citizens to avoid going out of your homes on Friday for your own safety."

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