Hundreds of truckloads of fighters from different militias left the city of Misurata, heading to Tripoli to help fend off Hifter's forces, said militia sources and residents of Misurata, about 195km east of the capital.
Many Misurata residents - and its militias - despise Hifter and view him as another dictator in the making. Militias from other pro-government cities such as Zintan also moved into Tripoli, according to photos posted on social media.
In Misurata, a radio station sent out a rallying cry to its listeners: "Everyone who owns a gun please go to Tripoli right away to fight for your country against Hifter."
Fayez Sarraj, chairman of the Presidential Council of Libya, said Hifter rejected concessions offered "to avoid bloodshed."
"We were stabbed in the back," he said in televised comments, AP reported.
Tripoli is a city accustomed to eruptions of militia violence. In some neighbourhoods, life remained typical with people shopping and going to work or school, said residents reached by phone.
"The city is normal," said Jamal Mustafa, 35, an employee at a Libyan overseas investment firm. "People are shopping and going out and doing their routines."
But in neighbourhoods closer to the fighting, residents were preparing for the worst. Many remained inside their homes, as heavily armed militia vehicles steadily drove through to the front line.
Jamal Ramadan, 42, whose house is less than 5km from the old airport, decided to flee.
"We could hear heavy shelling and gunshots," said the taxi driver and father of three children, ages 3 to 6. "My wife told me she was to afraid to stay. So we got a few clothes and left. We are not going to gamble our lives on this."
They drove about 145km to stay with his wife's relatives.
"We don't want Hifter to come," said Mustafa. "Anyone who wants to rule the country like Gadaffi is unacceptable."
Human Rights Watch raised concerns of possible abuses happening if fighting escalated inside the capital. Activists accuse Hifter's fighters of committing numerous human rights violations, including summary executions, indiscriminate attacks on civilians and arbitrary detentions.
Pro-government militias also have a track record of abuses against civilians, the watchdog group said.
"Whenever rival armed forces clash in Libyan cities, it's civilians who suffer the most," Sarah Leah Whitson, the group's Middle East and North Africa director, said in a statement. "All sides need to abide by the laws and minimize civilian harm."
Earlier, government warplanes targeted Hifter's militiamen in attempts to stop his push toward Tripoli. The warplanes bombed positions of his self-described Libyan National Army that is south of Tripoli, prompting the warlord to declare that his forces would shoot down any aircraft flying over western Libya, local media reported. Tripoli residents on social media described hearing fighter jets passing over the city.
The aerial assault came a day after Hifter's forces were stopped from advancing in Tripoli at a strategic checkpoint and about 100 of his fighters were captured by the pro-government militias, local media reports said.
But Hifter's forces appeared to have regrouped. Their media office said on its Facebook page claimed they had not only seized control of the airport but also had captured another enclave, Wadi el-Rabeia, south of the capital.
There was no immediate response from the Tripoli government nor the militias that back it.
On Friday, Hifter ordered his forces to seize control of Tripoli following their takeover of Gharyan, a town roughly 95km south of the capital. UN Secretary-General António Guterres personally intervened in a bid to stop the fighting but failed to convince Hifter to halt his offensive.
Hifter's attempted power grab also risked setting off a fresh wave of people heading toward Libya's borders or attempting to reach Europe over dangerous sea routes in the Mediterranean. Fearing a spillover of refugees, neighbouring Tunisia has tightened control over its border.
Also of concern is that a power vacuum and more insecurity could allow the Isis affiliate, which once ruled the city of Sirte, to regroup.
The United Nations, the United States and other governments, including France and the United Arab Emirates which supports Hifter, have all demanded that he pull back his forces. Russian Foreign Minister Sergei Lavrov called for restraint from all sides at a news conference in Cairo.
Any escalation in fighting threatens to torpedo a UN-sponsored reconciliation conference to forge a path forward for elections, scheduled for next weekend.
But Ghassan Salame, the UN special envoy to Libya, said the conference would go forward as planned, declaring that it was a year in the making and that the world body would not quickly give up its political work.
After meeting with the Tripoli Government's president, Fayez Serraj, Salame said in a tweet: "I want to reassure the Libyans that the UN will not leave them by themselves & will stay in Libya, working toward a political solution, silencing the guns & a peaceful political understanding btwn the various parties"
