Henchi Ordjo, an opposition alliance spokesman, told Reuters news agency on Saturday that fighters had simply held back an assault on the palace to allow Idriss Deby, Chad's president, the opportunity to leave.
"No ceasefire has been agreed," Ordjo said.
Battle in capital
Al Jazeera's Haru Mutasa, reporting from Njadmena, said that a new round of fighting had broken out early on Sunday.
"It doesn't look like a ceasefire is in place, the fighting is getting closer to where we are," she said.
Deby has turned down a French offer to help him escape from the opposition forces' advance, an official source in Paris said told AFP news agency on Sunday.
Mutasa said that several Chadian soldiers had come to the gates of a hotel where expatriates are holed up in an effort to take refuge, but were turned away by French troops.
"No one knows who is in control, but there are reports that the government is sending in reinforcements to fight the rebels. This is all going on while the French army tries to evacuate French and other foreign nationals," Mutasa said.
She also said that while the evacuations of foreigners were under way, Chadian residents have been left to fend for themselves.
"It was planned that the Chadian army would also protect civilians, but it doesn't seem to be the case now - and they are left to find shelter from from the fighting."
There has been no official confirmation about the whereabouts of Deby, but two of his ministers said he remained inside the palace complex at the head of loyal troops.
Mahamat Ali Abdallah Nassour, minister of state in Deby's government, told Radio France International (RFI) that Chad's security forces were "in control in the capital".
Chadian army helicopters took off from the military base at the airport and had opened fire on opposition fighters in south Ndjamena, a military source said on Saturday.
A French air force plane began evacuating several hundred French and other foreign nationals to Gabon, an official at the French military base in Ndjamena said.
The US embassy said its non-essential staff and family members would also be evacuated.
Herve Morin, French defence minister, said France, Chad's former colonial power, which has been accused by the rebels of propping up Deby, would remain "neutral" in the conflict in the oil-producing central African state.
President Idriss Deby seized power in a Libyan-backed coup in 1990
He went on to win the Chad's first two
multi-party elections in 1996 and 2001
A ceasefire signed between Deby and four rebel groups in October recently collapsed
The largest rebel group, the Union of Forces for Democracy and Development, is led by a former minister who accuses Deby of corruption
Click here for more on Chad's spiral into conflict
But Bernard Kouchner, the French foreign minister, condemned what he called "a brutal attack against an elected and legitimate president".
He called for a ceasefire and negotiations.
The US and African Union also condemned the rebels' attack on Ndjamena.
The AU threatened to eject Chad from the 53-nation body if the opposition alliance took power.
The rebels allegedly met little resistance as they advanced across the country from the eastern border with Sudan's Darfur region.
Chad says the rebels are armed and backed by the Sudanese government. Khartoum denies such accusations.
A Chadian opposition web site, Alwihda, said civilians were fleeing the capital towards the border with Cameroon in the south.
Diplomats and residents said it was difficult to tell who controlled the city, where outbreaks of looting were reported.
"We're watching the ebb and flow of battle," one diplomat said, adding army soldiers and rebels on foot and in vehicles were moving around the capital.
This week's fighting delayed the deployment of a European Union peacekeeping force, Eufor, to eastern Chad.
The leaders of the Chadian rebels include Timane Erdimi, a former member of Deby's ruling clan, and Mahamat Nouri, a former defence minister.
They are among several high-level officials who have defected to the rebels in recent years, accusing Deby of ruling like a dictator and favouring his family and friends.
Deby was widely criticised after he changed the constitution in 2006 to enable him to hold the presidency for a third term.
In October, four Chadian rebel groups initialled a peace agreement with the government in a deal brokered by Libya.
In November, Nouri's UFDD rebel group abandoned a ceasefire and accused the government of not honouring parts of the Libyan-brokered peace accord.
Gaddafi and Denis Sassou-Nguesso, Congo's president, have been asked to monitor the situation in the landlocked country, according to the AU.