Smoke billowed over the city as protesters burned tyres and piled up stones to block the roads and prevent anyone from going to work.
The Hezbollah-led opposition called the action as part of a continuing campaign to force the government to resign.
The authorities have warned troops might be used to maintain order.
Protesters turned out early on Tuesday to cut off several streets in central Beirut, witnesses said.
The only road leading to Beirut's international airport was blocked, and several flights were cancelled as a result. Several main roads connecting the capital with other towns were also occupied.
Security forces were standing by, the BBC's Jim Muir in Lebanon says.
The government has said they will keep the roads open, but witnesses say little action has been taken so far.
At least three of the protesters were reported to have been wounded by gunfire when they tried to close the roads at Byblos, a mainly Christian port town in the north.
Demonstrators say they will keep up their protests until they achieve their aims.
Hezbollah wants the formation of a national unity government in which it and its allies have a big enough share to carry a veto.
Since 1 December, they have been besieging the main government building in Beirut.
Their tactics have yet to have the desired effect so they have decided to try to step up the pressure by calling this general strike, our correspondent says.
He says the strike call has pitched the country into a highly uncertain and very tense situation, with many unknown factors, not least whether this action will be open-ended or limited to one day.
Prime Minister Fouad Siniora says the army and security forces will take steps to ensure the roads stay open so people who want to go to work can do so.
With the government strongly urging people to go about their business as usual, there is no doubt this will be a trial of strength, our correspondent says.
It comes at a particularly damaging time for the government, as potential donors gather in Paris for a major aid conference on Thursday to help get Lebanon back on its feet after last summer's Hezbollah-Israel war.
The contest over the strike has strong sectarian overtones, our correspondent says.
It is almost certain to be observed in Shia areas, as it is supported by the two big Shia factions - Hezbollah and its mainstream ally, Amal.
The Sunnis and Druze largely support the government, so their regions are unlikely to take part.
But there will be confusion in mixed areas, and among the Christian community.
Michel Aoun and several other Christian factions support Hezbollah and the opposition, while other Christian groups back the government.Last Mod: 00 0000, 00:00