"We have taken steps similar to those of a state of emergency in order to deal with any possible repercussions," Interior Minister Hassen Sabeh was quoted by Lebanon's An-Nahar daily newspaper on Saturday as saying.
"It is normal to expect that the report will spark political and security reactions whatever its content."
Sabeh did not say what measures had been taken, but police and troops have increased patrols in the capital and surrounding areas.
A panel, headed by German prosecutor Detlev Mehlis, has been investigating the 14 February murder of al-Hariri in Beirut, widely blamed on Lebanon's neighbour and long time powerbroker, Syria.
Damascus has roundly denied the allegations.
Mehlis is due to submit his report to UN Secretary-General Kofi Annan and Lebanese officials on 21 October, three days before his commission's mandate expires.
In reaction to a domestic and international outcry over the murder, Syria withdrew its troops from Lebanon in April, and claimed that it had also pulled out all of its security agents.
Sinoria will go to Paris to discuss
the status of Palestinian militias
However, there have been a number of deadly bombings and attempted murders of anti-Syrian figures since then, and the government feels more might be in the offing.
Four current and former senior Lebanese security officials have already been detained in connection with the investigation, which included the questioning of a number of senior Syrian officials.
One official who was questioned, Interior Minister Ghazi Kanaan, who headed Syrian military intelligence in Lebanon for 20 years, was found dead in his Damascus office on Wednesday.
An inquest said he shot himself, but sceptics say he might have been murdered to keep him from revealing what he might have known about al-Hariri case.
Future status of militias
Meanwhile, Lebanese Prime Minister Fuad Siniora is to travel to Paris on Monday for talks with French and UN officials on the future status of home grown and Palestinian militias in his country, an official said on Saturday.
Siniora will also meet Palestinian leader Mahmud Abbas, who will be in the French capital as part of a wider European tour.
"It is normal to expect that the report will spark political and security reactions whatever its content"
His talks with UN envoy Terje Roed-Larsen will focus on steps taken to implement Security Council Resolution 1559, adopted in September 2004.
It called for the withdrawal of all foreign troops and the disbanding and disarmament of Lebanon's Hizb Allah and the 12 Palestinian groups present within and outside refugee camps.
Siniora said earlier this month that the presence of militia bases outside the camps would no longer be tolerated.
According to a tacit agreement after the collapse in 1987 of the Cairo accords that regulated Palestinian armed presence, the army refrains from entering the refugee camps, where carrying light weapons is permitted.