With his once iron grip on Peru crippled during a roiling political crisis, President Alberto Fujimori said today he will step down within 48 hours by sending a formal letter of resignation to the Peruvian Congress from Japan.

In a brief statement issued in Tokyo, where he arrived Friday, Fujimori confirmed an earlier announcement by the Peruvian government that he would resign, after 10 tumultuous years in office.

Fujimori, South America's longest-ruling leader, did not say whether he will return to Peru. Japan is his parents' native country and the home of his son and other relatives. His decision to resign an ocean away, while characteristically unpredictable, was assailed by his allies, including his own cabinet ministers. They saw it as contrary to the image of a leader who had long been known for his take-charge, bold and confrontational style.

Under pressure from corruption scandals that stung his administration in September, Fujimori, 62, had already called an early election for April 8 and promised not to run in it. Today's statement said he decided to step down "after evaluating . . . the new composition of the forces in Congress," a reference to the opposition's success last week in gaining control of the legislature for the first time in eight years.

Though Peruvians appeared to react calmly to the announcement, it was not clear who would succeed Fujimori. The constitution provides for First Vice President Francisco Tudela, Peru's former ambassador to the United Nations, to take office, but it wasn't certain he would do so. In October he said he was resigning, but Congress never approved the resignation. Also, Tudela might resist becoming interim president because that would prevent him from running for president in April.

Second Vice President Ricardo Marquez would be next in line, and he said he would take the job if Tudela doesn't get it. However, opposition leader Alejandro Toledo said neither Tudela nor Marquez should be president, because both were elected in the same controversial runoff that Fujimori won in May, a vote that drew widespread fraud allegations. Toledo called for the new president of Congress, opposition member Valentin Paniagua, to take over.

Peru's powerful armed forces said they would support any changes in the government as long as the constitution was respected.

The political crisis in this Andean nation of 25 million has attracted international attention because the country is seen as a test case for the establishment of stable democracies in South America. Fujimori is the foremost example of a new breed of democratically elected yet authoritarian leaders who have come to power in Peru, Venezuela and Bolivia.

Fujimori had been scheduled to remain as president until July 28, when the president elected in April is to take office. His decision to resign was announced in Lima by Prime Minister Federico Salas and confirmed in a statement issued by a Peruvian Embassy official in Tokyo.

Neither statement indicated whether Fujimori will return to Peru. Japanese officials have denied speculation in Peru that Fujimori is seeking asylum to protect against prosecution for corruption scandals. His son, sister and brother-in-law, Peru's ambassador to Japan, live in Tokyo.

High-ranking U.S. officials were informed by Peruvian authorities this morning that Fujimori's stay in Tokyo will be "indefinite."

Fujimori's cabinet issued a statement expressing the members' "indignation" that the resignation announcement was "made outside the country in the context of a grave crisis and uncertainty about [Fujimori's] return." Economy Minister Carlos Bolona said Fujimori "jumped ship."

With the opposition taking over in Congress, calls had been building for Fujimori to step down early. Also looming was an attempt by his opponents in Congress to oust him from office on the grounds of "moral incapacity" and investigations into government corruption charges.

Effectively reined in by the Congress he once controlled and without the solid backing of his military, Fujimori had become largely superfluous in Peruvian politics--even ridiculed by opponents who had begun distributing trash bags and toilet paper rolls with his picture on them.

In an audacious, authoritarian decade in power, the former university rector was often sustained by his popularity among the people, fueled by his success in crushing two powerful guerrilla movements and licking hyperinflation. But his popular support fell as scandals erupted and a one-month hunt for his reviled ex-spy chief, Vladimir Montesinos, proved unsuccessful.

Montesinos, long the power broker behind Fujimori, was at the center of the scandals that forced the president to call new elections. In September Montesinos was shown on videotape paying a $15,000 bribe to an opposition congressman. Montesinos was also linked to an arms-for-drugs deal with Colombian guerrillas.

Fujimori called the early election against Montesinos's wishes, prompting a power struggle between the two. Montesinos mounted what diplomats have called an attempt to take power and fled to Panama when it failed. He was not able to win asylum there, and returned to Peru in October. Since then he has been in hiding.

As news emerged in recent weeks that Montesinos had more than $50 million stashed in secret bank accounts around the world, opinion polls showed that two-thirds of Peruvians believed Fujimori was aware of the corruption--something the president had denied.

Three weeks ago, Fujimori used what little clout he had left with the armed forces to oust the military leaders who had been loyal to Montesinos. But the move, along with his inability to capture Montesinos, left Fujimori politically weak.

"He had become a lame duck president with nothing to gain by staying in power--except a headache," said one high-ranking U.S. official.

Marquez, the second vice president, said today that if Tudela "remains firm in his decision to resign as vice president," then he, Marquez, was "willing to assume the presidency to guarantee a clean, transparent transition and electoral process." But opposition leader Toledo said that "in order to give Peru the necessary confidence, the head of Congress should assume the presidency."

The president of Congress, Paniagua, is an elder statesman who is not viewed as overly partisan. The door was also open to some other kind of transition government, jointly agreed to by the government and opposition.

"I think this is more of a relief than a tragedy for the nation," said Francisco Loayza, a Lima-based military and political analyst. "Fujimori may have been a stabilizing force before, but the uncertainty about his ability and desire to rule was creating an even greater crisis. This is, in some ways, resolution."

But Fujimori's decision also essentially leaves the task of restoring order to the country's divided opposition, whose leading members are now preparing their own election bids. "They will have to rise to the occasion now," Loayza said.

In a statement faxed to Agence France-Presse Friday, Fujimori had been vague about his plans to remain in office until July but also said he would like to run for Congress in Peru.

Though Japan rarely grants asylum, there was a sense among some diplomats that Fujimori's family background would help him if that is his objective. Perhaps even more important is that in 1997, Fujimori staged a risky mission to free hostages held by leftist guerrillas at the Japanese ambassador's residence in Lima. Among the dignitaries who were freed unharmed was Japan's ambassador to Peru.

"They may owe him one," said one Western diplomat.