Gunmen fired on protesters trying to block roads, wounding at least 12 people.
The only road leading to Beirut's international airport was among those cut off. Meanwhile, several Arab and international airlines were reportedly suspending flights to Beirut.
The strike is led by the Shi'ite Hizballah movement, which seeks the formation of a national-unity government in which it and its allies have a big enough share to carry a veto. Siniora's Western-backed government has shrugged off the demands.