Lebanese troops have been deployed in the capital and other key cities a day before the release of a UN report into the killing of former PM Rafik Hariri.
The Lebanese army, ambulances and fire service are out in force
Armoured vehicles and soldiers have been patrolling Beirut, in what the interior minister described as an unofficial state of emergency.
A UN team has been investigating the assassination of Mr Hariri in a devastating bombing in February.
Many in Lebanon blamed Syria, which is something Damascus denied repeatedly.
The report was handed over to UN chief Kofi Annan on Thursday.
The UN Security Council will see the report on Friday and be briefed on 25 October by the team's leader, German magistrate Detlev Mehlis.
Lebanese Prime Minister Fouad Siniora said that while he did not know what was in the report, Thursday's measures had been taken in case it revealed who was behind the murder.
BBC Beirut correspondent Kim Ghattas says the army has been deployed around Beirut and its surroundings, as well as other major cities.
If Mehlis' report names Hariri's killers, Lebanese officials fear violence
Ambulances and fire engines are on standby on key intersections around the capital.
The UN has also taken measures to protect its personnel in Lebanon because of what it called the "highly energised" situation in Beirut.
Its offices in the capital have been fortified, and the head of the UN Information Centre, Nejib Friji, has been temporarily withdrawn from Beirut "for his own safety".
Since Mr Hariri was killed a series of bomb attacks have targeted anti-Syrian journalists and politicians as well as Christian areas.
In a separate development, Mr Hariri's son, Saad, has said he wants those implicated by Mr Mehlis' report to be tried by an international court.
"We could not carry out the investigation and requested the help of the UN. Of course we will demand an international trial," he said after a meeting with the Arab League Secretary General Amr Moussa on Wednesday.
The blast in Beirut in February killed Rafik Hariri and 20 others
"Mehlis' report will be clear and we will find out who committed the crime."
Saad Hariri, who leads the largest bloc in the Lebanese parliament, also said that countries should not be allowed to exploit the fallout from the publication of the investigation's findings for their own interests.
Syria, the main power in Lebanon until its military withdrawal earlier this year, denies any involvement in the killing of Rafik Hariri.
"We are 100% innocent," Syrian President Bashar al-Assad told German weekly newspaper Die Zeit on Wednesday.
However, Mr Mehlis has listed four pro-Syrian Lebanese generals as suspects, and questioned seven Syrian officials, including Interior Minister Ghazi Kanaan, who was found dead last week after apparently committing suicide.
The generals were arrested in Lebanon in August.
Our correspondent says many Lebanese are now hoping that the publication of the UN report will finally satisfy their desire to know the truth about who killed Hariri and put an end to feelings of insecurity here.
But there are fears that the UN report could well add to the political turmoil, our correspondent adds.