Lebanon's army has been put on high alert after former Prime Minister Rafik Hariri was killed in an apparent assassination on Monday.
The car bomb in the centre of Beirut left at least nine others dead and caused widespread destruction.
An army statement said it had "ordered a general mobilisation of all army units, raised to the maximum its state of alert" to "safeguard stability".
Three days of mourning have begun in a country that observers say is in shock.
The killing comes at a time of rising tension between Syria, Lebanon's political master, and members of the opposition, BBC correspondents say.
Lebanese opposition leaders have said they hold the Lebanese and Syrian governments responsible for the killing.
Angry mourners in Beirut tried to set fire to the local offices of the Syrian Baath party.
The opposition also called for the government's resignation, for Syrian troops to withdraw from the country before elections in May and for a three-day strike.
Syrian President Bashar al-Assad condemned Monday's attack as a "terrible criminal act".
The White House described it as a "brutal" murder and said Lebanon should be allowed to pursue its political future "free from violence... and free from Syrian occupation".
But Syria's ambassador in Washington, Imad Moustapha, said that the US administration's reaction was not helpful.
"Trying to score politically when there is a tragic event like this is not really constructive," he told CNN.
Lebanon's former colonial power France, whose leader President Jacques Chirac had close ties with Mr Hariri, called for an international inquiry into the blast.
Street patrols
The bomb exploded as the former prime minister's motorcade passed the derelict St Georges Hotel, near the city's harbour, and is thought to have killed some members of the convoy.
Among about 100 people injured was at least one former minister who, along with Mr Hariri, had just attended a session in parliament.
In a statement, Lebanon's army said its "fighting readiness was raised to the highest degree" in response to the bomb.
"The army has taken preventative measures in various regions in order to safeguard stability, has deployed patrols in the streets of the capital and other regions and set up checkpoints," it went on.
Soldiers on leave have been recalled and all holiday has been suspended, the army leadership said.
After the attack, supporters of Mr Hariri took to the streets of Beirut and in his home town of Sidon, south east Lebanon, people burned tyres in protest.
The BBC's Kim Ghattas in Beirut says it is still unclear what caused the blast.
A little-known group calling itself Victory and Jihad in Greater Syria issued a statement claiming responsibility and saying it was a suicide bomb, but the claim cannot be verified, our correspondent says.
Local television stations broadcast pictures of a vast bomb site, with cars ablaze and buildings seriously damaged.
PHOTO CAPTION
A wounded man lying on the ground asks for help after a massive car bomb explosion in Beirut February 14, 2005. (REUTERS)