The fighting in Tripoli comes after pro- and anti-government fighters traded gunfire in Beirut, Lebanon's capital.
James Bays, Al Jazeera's correspondent in Beirut, said that intense gunfire was heard in the commercial area of Hamra in the western part of the city shortly after midnight.
He said that the capital was relatively calm by mid-morning.
Also on Monday, paramedics told the Associated Press that at least 11 people had been killed and 20 wounded in clashes between rival armed groups in Chouweifat, a mountain town near Beirut.
Bays reported that it was uncertain whether the overnight clashes in Beirut represented an isolated incident or whether they would lead to widespread fighting.
He said the clashes occurred near the home of Saad al-Hariri, a Sunni Muslim leader.
Al-Hariri is a son of Rafiq al-Hariri, a former Lebanese prime minister who was assassinated three years ago.
A two-man Al Jazeera camera crew suffered injuries while trying to film the fighting and were evacuated by the Lebanese army.
The fighting has raised fears that Lebanon could descend into civil war.
Sunday had largely been quiet in Beirut after four days of fighting in the city.
The Lebanese capital was the focus of four days of clashes that intensified with Hezbollah fighters seizing large swaths of west Beirut - demonstrating its military might in a showdown with the government.
Fighting first broke out days ago after the Lebanese government - which is boycotted by the opposition - ordered the closure of a private telecommunications network owned by Hezbollah, a Shia opposition organisation.
The government also suspended Beirut airport's head of security over alleged ties to Hezbollah.
After opposition forces took control of west Beirut, the army said it would not implement the government's decisions, for fear that it could inflame sectarian tensions.
The opposition, which has now stood down its forces in Beirut to allow the army to maintain security, has said it reserves the right to lead a campaign of civil disobedience until its political demands are met.
The government has not yet decided whether to rescind its decisions on the communications network and airport chief.
Amine Gemayel, leader of al-Kataeb, a Maronite Christian party, called on Monday for Hezbollah to refrain from using its arms against Lebanese.
"We want a pledge from Hassan Nasrallah personally in front of Arab, international, Islamic and Christian public opinion - in front of Iran, Syria, Saudi Arabia and all the others ... a guaranteed pledge that weapons will never again be used in Lebanon for political purposes," he said.
"What is the meaning of any dialogue under the threat of weapons?"
The fighting has not just been confined to the capital.
The clashes began in the northern city of Tripoli on Sunday, where opposition supporters clashed with those allied to the government of Siniora.
Fighting also spread to the east, engulfing Mount Lebanon, just outside Beirut.
The clashes resulted in the collapse of pro-government forces in the Aley region, a stronghold of Walid Jumblatt, a prominent pro-government politician and a leader of Lebanon's Druze community.
The area had been controlled mostly by Jumblatt's Progressive Socialist Party (PSP) and its fighters.
Reporting from Mount Lebanon, Al Jazeera's Zeina Khodr said on Monday that pro- and anti-government supporters were accusing each other of starting the violence there.
"People here are angry and nervous," she said.
She reported that the violence on Mount Lebanon was largely between Druze and Shia Muslim fighters, and that the clashes appeared to be part of a wider battle for control of the area.
"The Mount Lebanon region is a strategic area – it links Beirut to Syria, it links Beirut to south Lebanon and the [eastern] Bekaa valley," Khodr said.
Jumblatt tacitly conceded the loss of his stronghold on Sunday and urged Talal Arslan, a Druze rival allied with the Hezbollah-led opposition, to place the affected regions under army rule.
He said: "Things [have] deteriorated in the area of Aley in such a way that is threatening the co-existence with our Shia brothers.
"I agreed with [Nabih] Berri [the parliamentary speaker] to entrust Talal Arslan with seeking to put an end to all the fighting.
"I say to my supporters that civil peace, co-existence and ending the destruction [must] prevail over all the other considerations."
On the diplomatic front, Arab foreign ministers said after crisis talks in Cairo that they will send a high-level delegation to Beirut for dialogue with political leaders.
A resolution issued at the end of Sunday's meeting urged Lebanese politicians "to attend a meeting with a ministerial delegation ... in order to discuss the dangerous situation in Lebanon and draw up an urgent road map to implement the Arab initiative".
The initiative aims to bring together three opposition leaders (Berri, the parliament speaker; Michel Aoun, a Maronite Christian politician; and Hassan Nasrallah, the Hezbollah secretary-general) and three government figures: Siniora, al-Hariri, the parliamentary majority leader, and Amin Gemayel, a former president.
A delegation headed by Qatar and including Algeria, Djibouti, Jordan, Morocco, the United Arab Emirates, Yemen and Amr Moussa, the Arab League secretary-general, will head to Lebanon "very soon" for the talks, Moussa said.
The mission will deliberately exclude Saudi Arabia and Egypt, which have been vocal in their backing of Siniora's government, as well as Syria, which backs Hezbollah.
Source: Al Jazeera and agencies