Fighters hoping to install Ivory Coast's democratically elected president descended on the country's largest city, where forces have reportedly attacked the residence of entrenched ruler Laurent Gbagbo on Friday.
A spokesman loyal to Alessane Ouattara, the U.N.-recognized leader, told Reuters that fighters seized control of state broadcaster RTI, which went off-air late Thursday.
Gbagbo's camp offered no immediate response.
Forces backing Alassane Ouattara have overrun nearly 80 percent of the country as soldiers fled and towns fell in quick succession. The regular army put up almost no resistance during the four-day offensive, including in the ruler's hometown, where fighters broke into Gbagbo's compound and slept in his bed.
But they may face fierce resistance on the peninsula where the presidential palace is located, surrounded on all sides by a natural moat -- Abidjan's glassy lagoon.
Gunfire could be heard throughout the day, along with the concussive boom of heavy artillery. Reporters saw soldiers in camouflage race across the waterside highway in pickup trucks mounted with machine guns.
As his forces amassed on the outskirts of Abidjan, Ouattara made a final appeal to Gbagbo to step down, and called on the rest of the army to defect.
"(My fighters) have come to restore democracy and ensure respect of the vote by the people ...Today they are at the doorstep of Abidjan," Ouattara said in an address broadcast on his private TV station.
"To all those who are still hesitating, whether you are generals, superior officers, officers, sub officers, rank-and-file ... there is still time to join your brothers-in-arms," he said.
As the columns of pro-Ouattara forces advanced, the head of the army, Gen. Phillippe Mangou, sought refuge at the home of the South African ambassador in Abidjan with his wife and five children, South Africa's foreign ministry said in a statement.
By mid-afternoon, as many as 50,000 soldiers, police and gendarmes had abandoned Gbagbo, according to the head of the United Nations mission, Choi Young-jin. "Only the Republican Guard and his special forces have remained loyal," guarding the palace and residence, he told France-Info.
Ouattara was declared the winner of last November's presidential election by the country's election commission in results verified by international observers. But after a decade in power, Gbagbo refused to accept his loss. He has used the military to attack pro-Ouattara areas with heavy-artillery and is accused of arming citizen militias and recruiting foreign mercenaries to defend his grip on power.
Up to 1 million people have fled the fighting and at least 490 people have been killed since the election, most of them supporters of Ouattara.
Gbagbo hasn't been seen in public since this week's military offensive began, even though state TV announced Wednesday that he was preparing to address the nation.
Those who know him well say even an armed onslaught will not make Gbagbo cede power.
"He has no intention of resigning," said one of his advisers in Europe, Toussaint Alain. "He will not resign in the wake of this attack. He is not going to abdicate. He is not going to lay down his arms. He will stay in power to lead the resistance to this attack against Ivory Coast."
However, a senior diplomat who has been in contact with members of Gbagbo's inner circle said a standoff appeared to be building between hard-liners who want Gbagbo to fight to the end, and others who are urging him to step down. The diplomat spoke on condition of anonymity because he is not authorized to speak to the media.
It is not clear what the pro-Ouattara fighters will do if they manage to reach the presidential palace. Ouattara's spokesman refused to speculate, but said the use of force is necessary because Gbagbo has frustrated all attempts to find a diplomatic solution.
"The end is almost here. It's a matter of hours," said the spokesman, Patrick Achi. "We issued our ultimatum yesterday ... If Gbagbo does not want the fighting to happen in Abidjan, he should surrender. If he doesn't, we have no choice."
In the four months since the disputed election, the international community has repeatedly offered Gbagbo a golden parachute, only to be rebuffed. He twice refused to take a phone call from President Barack Obama, who offered him a teaching position at a Boston university if he agreed to peacefully step aside.
During this time, Ouattara pleaded with world leaders asking for a military intervention to oust the defiant leader. Although the United Nations passed resolutions allowing their peacekeepers to intervene to protect civilians, pro-Ouattara neighborhoods like Abobo and Anyama continued to be pummeled with mortars. So many people were killed the local morgue began stacking corpses on the floor because they had run out of space in the refrigerated vaults.
On Thursday, Secretary-General Ban Ki-moon reiterated his demand that Gbagbo immediately cede power to Ouattara "to enable the full transition of state institutions to the legitimate authorities," said spokesman Farhan Haq.
In Washington, the top American diplomat for Africa, Johnnie Carson, said Gbagbo needed to seize this last opportunity to step aside.
"There is a clear indication that the military forces of Gbagbo have started to disintegrate," he said. "The rapid pace at which Alassane Ouattara's forces have been able to move across the country from east to west and up to Abidjan suggests that there have been widespread desertions in the Gbagbo forces."
Ouattara's fighters are largely drawn from a rebel group based in the country's north that launched a 2002 rebellion against Gbagbo. For over three months, Ouattara refused to allow them to march on Abidjan.
The advance was a last resort after all other diplomatic means had failed, say Ouattara's supporters. Ouattara won the election with over 54 percent of the vote and did not want to be seen as having taken the country by force.
The Associated Press contributed to this report.