A woman, who declined to give her name, said all the main roads in Hadba al-Khadra were closed and that residents were staying indoors.
It was not immediately clear who was responsible for the clashes, but Tripoli is rife with heavily armed militias.
The new fighting came hours after two powerful armed groups, Thowar Tarabulus and the Abu Slim Brigade, denounced the ceasefire announced by the UN-backed Government of National Accord (GNA).
Thowar Tarabulus controls the city centre of Tripoli while Abu Slim controls the southern working-class district of the same name.
Both groups denied agreeing to the truce that put an end to four days of fierce fighting in Tripoli.
The city had been paralysed amid exchanges of rocket and artillery fire between pro-unity government forces and rival militias including groups allied with former prime minister Khalifa Ghweil.
Clashes on Wednesday night rocked the southern Salaheddine district, where several rival militias occupy barracks.
The truce deal -- signed by the GNA, local mayors and militias from Tripoli and third city Mistrata, was aimed at cementing the UN-backed government's control over large parts of the capital.
It provided for an "immediate ceasefire" and called for armed groups that do not recognise the GNA to leave Tripoli within 30 days.
It also demanded the release of people arrested since Monday, the GNA's defence ministry said.
But Thowar Tarabulus, in a statement on their Facebook page, said the "agreement signed in the presence of the presidential council does not represent us".
The Abu Slim Brigade said, in a separate online statement, "we did not sign the (ceasefire) agreement... we will only accept full solutions".
Militias have been key power brokers in a country plagued by violence and lawlessness since the NATO-backed ouster of longtime dictator Muammar Gaddafi in 2011.
The battle, triggered Monday night after the killing of a bank guard, saw pro-GNA forces expand their clout in the capital.
They took several districts from rival militias including groups allied with Ghweil, who was forced from power when the GNA arrived in Tripoli.
The GNA operation appeared to be well-prepared and coordinated. Thursday's deal charges GNA forces with securing areas controlled by rival groups.
Observers said it was in line with an inter-Libyan political deal backed by the UN and signed in December 2015.
That agreement, which gave rise to the GNA, called for armed groups to leave Tripoli and other Libyan towns.
But the capital remains home to dozens of militias. Since taking office, the unity government has secured the backing of several, but many parts of Tripoli remain out of its control.
The GNA wants to see heavy weapons withdrawn to allow its security forces to operate effectively.