Hizbullah supporters and government backers clashed in Beirut neighborhoods Wednesday, exchanging gunfire and throwing stones after the Shiite militant group blocked major roads to enforce an anti-government strike. The violence deepened tensions in a country already mired in a 17-month-old political crisis pitting the Iranian- and Syrian-backed Hizbullah against the government of US-backed Prime Minister Fuad Saniora. The troubles have left Lebanon without a president since November. Explosions and gunfire echoed throughout the city. The cause of the explosions was not immediately known, but witnesses and television reports said they may have been rocket-propelled grenades. Both sides accused each other of using gunfire in the clashes. An Associated Press photographer saw gunmen from Hizbullah and its allied Shiite Amal group shooting toward a building housing an office for a pro-government Sunni group. Police also were seen firing toward a building and at times, police fired in the air to try to disperse clashes.