A strong army presence was reported in Beirut, the northern port city of Tripoli and the Chouf mountains, where gun battles had raged between pro-government and Hezbollah-led opposition militias since May 7.
Police figures put the casualty toll at 62 people dead and almost 200 injured.
However, the actual figure was believed to be higher in the worst strife the country has seen since the end of the 1975-1990 civil war.
In a Monday evening statement, the army command said that as of Tuesday 6:00 a.m. local time, its troops would use all means necessary, including force, to end the violence and all armed conflicts by the rival forces.
In its most forceful move yet, the military measure came after implicit criticism by the pro-Western ruling March 14 coalition that the army was not doing enough to end the opposition's violent campaign in west Beirut and other areas.
Prime Minister Fouad Siniora on Saturday appealed to the army to restore law and order, saying he had urged "the army to live up to its national responsibilities without hesitation or delay, and this has not happened until now."
The army, trying to establish itself as an independent, professional and national institution after the civil war, remained neutral and has resisted being pulled into the political crisis that continued to deteriorate since six cabinet ministers – five of them Shiites – resigned in November 2006.
The military, made up of soldiers from all Lebanon's multi-sectarian fabric, has refused to engage in the fighting in order to maintain its unity and to remain the only institution that the rival pro- and anti-Western camps support and respect as the only element that can avert a total collapse.
But since the fighting erupted, the army has walked a very thin line as it tries to protect government officials and their supporters, while at the same time avoid provoking the opposition, especially the Shiite Hezbollah organization.
Hezbollah and its allies in the Shiite Amal movement, headed by House speaker Nabih Berri, began to withdraw their gunmen from the streets of Beirut on Sunday only after the army suspended two cabinet decisions that had triggered Hezbollah's wrath.
The cabinet last week had declared Hezbollah's telecommunications network illegal and dismissed the head of Beirut's airport security for his sympathies toward Hezbollah, moves that the organization regarded as a government "declaration of war" against it.
After fierce battles with pro-government gunmen, the opposition militia quickly handed to the army their captors, weapons, and the offices they raided. This led to some critics saying the military was in collusion with Hezbollah – backed by Iran and Syria.
But independent analysts said the army could not have done it any other way if it wanted to prevent the many Shiites in its ranks from deserting and risking a split in the army.
Besides, they added, the army has no desire to confront Hezbollah whose military wing is confident and armed to the teeth, after having liberated south Lebanon from Israeli occupation in 2000 and fiercely resisting during the Israeli war in summer 2006.
This policy may also explain why the army has not attempted to remove the opposition's barricades blocking the main highways, including the road leading to Lebanon's only international airport, which also remained shut on Tuesday.
The opposition's roadblocks are part of what they call its "civil disobedience campaign," which started in December 2006 with sit-in protest tents in the trendy downtown area of the capital demanding what it sees as "fair power-sharing" in the government.
The opposition says its campaign will only end when Siniora's government publicly revokes its latest decisions and engages in dialogue to resolve the country's crisis.
Although both sides of the divide have agreed on army chief General Michel Suleiman as the "consensus" presidential candidate to fill the vacant post since last November, they have failed to convene 19 parliament sessions to vote as they wrangle over the make-up of the next government.
The last session, scheduled for Tuesday, was called off due to the violence that has widened the gap between them.
Meanwhile, U.S. President George W. Bush said on Monday that Washington would ensure the Lebanese military "has the practical equipment" needed to act against Hezbollah.
Bush told the BBC's Arabic TV channel that Hezbollah was "moving against the Lebanese people." He added that supporting the Lebanese army against the organization "should send a signal to everybody that they [Hezbollah] are a destabilizing force," along with Iran.
The U.S. president, speaking on the eve of a tour to the Middle East, said he would focus on Lebanon in his talks with Arab leaders in Saudi Arabia and Egypt – two countries that have made their position clear against Hezbollah.
Although the Lebanese army is in need of equipment, analysts say, it will accept U.S. military assistance only to maintain peace, not on condition of acting against Hezbollah.
Hezbollah has strongly warned the Siniora government against seeking U.S. support and is said to be alert and prepared to engage any intervention by the U.S. guided missile destroyer, the USS Cole, which is positioned in the eastern Mediterranean off the Lebanese shores.