The Lebanese army has so far not intervened in the clashes
The Lebanese army has said it is prepared to use force to disarm gunmen and restore order across the country.
It follows a week of clashes between supporters of the Western-backed government and the Hezbollah-led opposition.
The latest violence has left more than 60 people dead.
Saudi Arabia has criticised Iran for supporting what it called a coup by Hezbollah. It said it would affect its relations with Arab countries.
There is now a strong Lebanese army presence in Beirut, the northern city of Tripoli and in the Chouf mountains south-east of the capital.
But correspondents say much of Beirut remains paralysed by street barricades, and the international airport is still closed.
The recent sectarian violence is the worst since the end of the 15-year civil war in 1990 and has led to fears of another all-out conflict.
A military statement said: "Army units will prevent any violations, whether by individuals or groups, in accordance with the law even if this is going to lead to the use of force."
Saudi Arabia, a strong supporter of the Lebanese government, has called for all Middle Eastern states to respect the independence of Lebanon.
"For Iran to back the coup that happened in Lebanon will have an impact on its relations with all Arab countries," the Saudi Foreign Minister Prince Saud al-Faisal warned on Tuesday.
In Tehran, Iranian President Mahmoud Ahmadinejad denied his country was meddling in Lebanon.
"Iran is the only country not interfering," he told a news conference.
The recent unrest began after the government moved to shut down Hezbollah's telecoms network and remove the chief of security at Beirut's airport for allegedly sympathising with Hezbollah.
The army has stayed neutral in the conflict emerging as the only factor preventing a complete collapse in Lebanon.
It is generally agreed that its commander, General Michel Suleiman, should be the next president.
For the past 16 months, Lebanon has been locked in political stalemate between the ruling coalition and Hezbollah-led opposition over the make-up of the government.
Lebanon has not had a president since November, when Emile Lahoud stepped down despite parliament failing to agree on his successor.
Lebanon was plunged into civil war from 1975-90, drawing in Syria and Israel. Many Lebanese are now wondering where the latest crisis will go from here.
An Arab League delegation trying to mediate an end to the fighting is expected to arrive in Lebanon on Wednesday.
President Bush on his Middle East solution
The US President George W Bush has also said he will discuss developments when he visits the region this week - visiting Saudi Arabia and Egypt.
In an interview with the BBC he said he had offered to help strengthen Lebanon's army so it can disarm Hezbollah.
The US is also sending its guided missile destroyer the USS Cole back into the Eastern Mediterranean.
Its action has angered Lebanon's Hezbollah-led opposition.
Sources have warned that any hint of American intervention would lead it to abandon the few red lines it has observed in its campaign to undermine the government.