DAKAR, Senegal — A rebel army swarmed the capital of Chad on Saturday, and gun battles erupted around the presidential palace, according to Chadian and Western officials, in an attack that raised the specter of deeper chaos in one the most war-scarred and fragile regions of the world.

Forces from a coalition of three rebel groups that have taken shelter in Sudan for the past few years entered the capital, Ndjamena, early Saturday after days of battle dozens of miles outside the city, Chadian officials said. The suddenness and stealth of their arrival appeared to take the military by surprise.

A spokesman for the three rebel groups, Abdraman Koulamallah, said in a statement posted on a rebel Web site that they were in the capital and were “ready to facilitate, with the guarantee of the African Union, the negotiated departure of President Idriss Déby and avoid a pointless blood bath.”

But Chad’s ambassador in Washington, Mahamoud Adam Bechir, said in a telephone interview that the rebels who had reached the capital were a small group that split from the main column of rebels headed toward the city. The group had circumvented counterattacks by the Chadian military and stolen into the capital, Mr. Bechir said, but was being chased by Presidential Guard forces.

“They were able to infiltrate the capital, panic the population, fire at the presidency and give the impression there is fighting going on at the presidency,” Mr. Bechir said. “But everything is under control. President Idriss Déby is in the palace. The Chadian military forces are chasing the insurgents.”

He said that the airport had been closed to civilian flights and that cellphone networks had been shut down to hamper rebel communication lines. As a result, his account of the fighting could not be verified.

The timing of the attack appeared to be linked to the planned arrival of a European Union force that was to be deployed on the border in an effort to protect refugees from the Darfur region of Sudan and from eastern Chad and to prevent Chad from sliding into bloodshed, said Reed Brody, a lawyer at Human Rights Watch who has studied Chad for many years.

A vast, arid and landlocked nation in the heart of Africa, Chad has suffered through years of civil war, military coups and tyrannical rule. But with the crisis on its eastern border with Darfur and conflict over a booming oil business in the south, the country has become increasingly unstable.

Ndjamena was plunged into confusion Saturday, with gunfire echoing through the streets while residents hunkered down in their homes, waiting for news. The United States, France and the United Nations made preparations to evacuate expatriates.

Gabriel Stauring, an American antigenocide activist, was among about 50 people pinned down in a luxury hotel in the capital that came under heavy fire. In an e-mail message, Mr. Stauring said that French military personnel had exchanged heavy fire with rebels outside the hotel.

“Bullets flew over our heads and parts of the walls and objects around us came raining down on us,” he wrote.

The fighting in Ndjamena will surely further destabilize what is already one of the most volatile regions of Africa. Chad and Sudan have traded accusations and bombs in the past four years as the conflagration in Darfur has increasingly consumed Chad as well.

Hundreds of thousands of Sudanese refugees from Darfur are living in Chad, and militia attacks from across the Sudanese border in 2006 forced tens of thousands of Chadians to flee their homes as well. Ethnic violence in Chad between Arab and non-Arab ethnic groups, echoes of the conflagration in Darfur, has forced still more to flee.

President Déby shares clan links to some of the leaders of the Darfur rebellion, and the rebels operate from bases in Chad with near total impunity, which has angered the Sudanese government and raised tensions between the countries.

Chad, meanwhile, accuses Sudan of sponsoring rebellions against Mr. Déby. The three groups that are attacking the capital all had bases in Sudan, according to analysts and diplomats, something that would be impossible without the tacit approval of the Sudanese government.

Mr. Brody, of Human Rights Watch, said that many Chadians feared a violent takeover by a shadowy group of rebels, many of whom have ties to repressive past regimes. “Nobody is going to miss Déby, but these guys aren’t exactly fighting for freedom and democracy,” Mr. Brody said.

In the past, France, the former colonial power in Chad, has used its military forces in Chad to bolster Mr. Déby. But on Saturday, French troops were focused on protecting expatriates, said Capt. Christophe Prazuck, a spokesman for the French military.

“At the present time, the French military forces are not involved in the fighting,” he said.

France maintains more than 1,200 troops in Chad, and on Friday and Saturday added 300 more to help protect its citizens, according to French officials. The United States State Department posted a message on its Web site urging Americans to seek safety at the embassy if they wished to be evacuated.

France’s foreign minister, Bernard Kouchner, went on television to condemn the rebel attack, and said President Nicolas Sarkozy spoke to Mr. Déby several times on Saturday. Speaking on a different television news channel in France, the defense minister, Hervé Morin, said France would remain neutral in the conflict.

The current fighting has led the European Union to delay its deployment of 3,700 peacekeepers to protect refugees living on borders of Chad and the Central African Republic.

The delay of that force is a blow to France’s ambitions to use European military power more forcefully, and senior French officials worked to keep other contributors on board.

“Politically it could be a little blow for our European operation in the eastern part of Chad,” a senior French official said. “The others are totally terrified.”