Thursday's gun battles broke out only minutes after the Hezbollah leader, Hassan Nasrallah, appeared on TV to accuse the Lebanese government of declaring war on his movement, whose raison d'etre is to resist Israel.
By Jim Muir
BBC News, Beirut
Lebanese soldiers blocked off roads in Beirut on Thursday
He said the only way out of the current crisis was for the government to rescind the decisions it took on Monday aimed at curbing Hezbollah.
The government has so far refused to do that but the Sunni leader, Saad Hariri, said that Hezbollah's reaction was based on a "big misunderstanding", and he offered direct talks to resolve the situation.
He appealed to Hassan Nasrallah to take a historic stand to "save Lebanon from hell".
But Hezbollah's TV station said the offer was rejected, and that the only acceptable solution was for the government to back down.
So the country has been left to the sound of gunfire, and very real fears that an unstoppable civil war could be starting.
The Lebanese army took the unusual step of warning that if the two sides did not stand back and seek compromise, its own unity could be threatened.
It is the last hope of many Lebanese for keeping the country together.
During the 15 years of civil war that started in 1975, the army split apart along sectarian lines under the strain.
Previous escalations in the current crisis, which has been building up for the last 18 months, have been headed off by cooperation between Saudi Arabia and Iran.
The Saudis are close to the Sunnis and the Beirut government, and Iran - to Hezbollah and the Shia.
But their own mutual relations seem currently to be troubled, and there has been no sign of decisive intervention by these influential outside powers to calm the situation.