At least eight people were reported wounded in clashes pitting mainly Sunni Muslim supporters of the Western-backed government against Shiite followers of the Hezbollah-led opposition.
In a tersely worded statement, the army command warned that "if this situation continues, everyone will lose and this will affect the unity of the military."
And newspapers drew parallels with the lead-up to the devastating 1975-1990 civil war.
The growing tension brought renewed expressions of support for Prime Minister Fuad Siniora from key Arab powers Egypt and Saudi Arabia, both of whom warned Hezbollah against any steps that might worsen the situation.
All eyes were on Hezbollah chief Hassan Nasrallah, who was due to hold a rare news conference later in the day.
Troops and riot police spread out in Beirut, with many schools and businesses remaining shut for a second straight day.
Armed men, some hooded or masked, were seen in several mixed Sunni and Shiite neighbourhoods.
Protesters burned tyres and lit fires along the airport road, which remained blocked by large mounds of earth dumped by Hezbollah supporters when a strike over wages on Wednesday degenerated into sectarian violence.
An official at the country's only international airport said all incoming and outgoing flights had been cancelled until at least 4:00 pm (1300 GMT), and it was unclear whether traffic would resume after that.
Government loyalists burned tyres and set up road blocks along various points of the main highway in the east of the country leading to Syria, forcing travellers to find alternate routes, an AFP correspondent witnessed.
Five people were wounded, four of them women, a security official in the eastern town of Shtura told AFP.
Three people were also wounded in the northern city of Tripoli in a shootout between rival factions.
The highway between the capital and the southern coastal city of Sidon was also closed by government supporters with burning tyres and piles of earth.
An official with Hezbollah ally Amal warned that the situation could get out of hand, and accused the majority of pushing the country toward a civil war.
"It is clear the majority is seeking an escalation and wants to push the country toward a civil war," the official told AFP. "What we are trying to do is calm down the situation."
As-Safir newspaper, close to the opposition, said Wednesday's incidents were a stark reminder for the Lebanese of the dark days of the 15-year civil war. They thought such incidents were a thing of the past and now new demarcation lines, with confessional tones, are emerging."
Al-Akhbar, another paper close to the opposition, said "the two camps have crossed all the red lines and are heading toward an armed confrontation."
Egypt's foreign minister repeated support for Siniora and implicitly blamed the opposition of "pushing for confrontation," the press reported on Wednesday.
"The party that is pushing for confrontation (the opposition) and which persists along this path with disregard for civil peace will surely bear the historic responsibility for its actions," he reportedly told Lebanese majority leader Saad Hariri in a telephone call.
Saudi Arabia also warned the opposition against an escalation.
"The kingdom urges the groups behind the escalation to reconsider their position, and to realise that leading Lebanon towards turmoil will not bring victory to any party except extremist external forces," the state news agency SPA quoted an official as saying.
The Syrian- and Iranian-backed opposition has vowed to keep up the protests until the government cancels decisions taken earlier in the week.
On Tuesday, the government said it was launching a probe into a private telephone network set up by Hezbollah, and accused the group of placing surveillance cameras around the airport to monitor the movements of pro-government politicians.
The cabinet also reassigned the head of airport security over allegations that he was close to Hezbollah.
The clashes erupted on Wednesday during what was supposed to be a general strike called by the main labour union over price increases and wage demands.
The political crisis, the worst since the civil war, has left the country without a president since November, when pro-Syrian Emile Lahoud stepped down at the end of his mandate with no elected successor.
© 2008 Agence France Presse