When acting Prime Minister Viktor S. Chernomyrdin arrived at the Russian White House in triumph last week to claim his office as head of the government, he was not the first to walk through the door. Striding in ahead of Chernomyrdin was the man who many believe put him there: tycoon Boris A. Berezovsky.

A billionaire banker who controls one of Russia's largest business empires, Berezovsky has emerged as one of the biggest winners in the high-stakes struggle for power in Russia. An adviser to Chernomyrdin and a longtime benefactor of President Boris N. Yeltsin and his family, he is regarded widely as the new power behind the throne."He has a very strong relationship with Yeltsin's circle," said former Deputy Prime Minister Boris Y. Nemtsov, a leading liberal who was ousted from his post last week along with Prime Minister Sergei V. Kiriyenko. "This pressure was organized by him and a very, very small group around him. Berezovsky is a modern Rasputin."

Like Rasputin - the mysterious monk who gained vast influence over Czar Nicholas II - Berezovsky seems to be the one capable of making or breaking top government officials. Moreover, the tycoon appears to be spearheading a move to nudge Yeltsin aside and transfer greater authority to Chernomyrdin.

Berezovsky, 52, who normally is not shy in talking to reporters, declined to discuss his role in the government shake-up. But he did take a shot at Nemtsov, a longtime antagonist.

"I can't treat seriously anything coming from Nemtsov because he is not a serious person himself," Berezovsky said. "He has certain problems with reaching maturity, a process that has been dragging on for too long, unfortunately."

Over the past week, Nemtsov, 38, repeatedly has charged that a cabal of oligarchs led by Berezovsky orchestrated Kiriyenko's removal because the government was on the verge of challenging their business empires. A major problem plaguing Russia - and a factor in the sudden collapse of the economy last week - has been the government's inability to collect taxes from big companies owned by some of the country's wealthiest men.

During his five months as prime minister, Kiriyenko had become increasingly aggressive in trying to extract tax payments from the rich. His government had drawn up a list of foundering banks and corporations to be declared bankrupt - including more than one controlled by Berezovsky - and the program was to be announced last Monday, Nemtsov has said. The plan was in line with conditions imposed by the International Monetary Fund and other foreign lenders this summer when they approved a $22.6-billion emergency loan package for Russia.

But on the day before the program was to take effect, Yeltsin suddenly announced he was sacking Kiriyenko and the entire Cabinet.

"The Kiriyenko government prepared a very strong program for restructuring the economy," said Nemtsov, who was in charge of economic policy in the Cabinet. "This program included bankruptcy of banks, even oligarchs' banks and the bankruptcy of big companies, some of them belonging to oligarchs. Of course, the oligarchs understood what we wanted to do and took part in our ouster."

To replace Kiriyenko, Yeltsin reinstated Chernomyrdin as prime minister. A wealthy businessman, he had served five years in the job before Yeltsin fired him in March. Chernomyrdin was Berezovsky's choice; the tycoon had endorsed him as the candidate to succeed Yeltsin as president in the next election.

With a doctorate in mathematics and physics, Berezovsky made his fortune shortly after the breakup of the Soviet Union by building the company LogoVaz into a nationwide car distributor.

Insiders say Berezovsky had been pushing for a government devaluation of the ruble, which was contrary to Yeltsin's policy but could have improved Berezovky's fortunes.

"Berezovsky argued for devaluation for a long time," said Vyacheslav Nikonov, a former Yelt-sin adviser.

Berezovsky widely is believed to have close relations to the president and his family - an idea he does not try to discourage. He was one of Yeltsin's major financial backers during the president's 1996 re-election campaign. He published Yeltsin's memoirs, providing the president with a steady source of income. He often gives jewelry to Yeltsin's wife and daughters, according to Alexander Korshakov, Yeltsin's former security chief. Yeltsin's son-in-law, Valery Okulov, received a job as general director of Aeroflot.

Berezovsky is also no stranger to government infighting, having served as deputy chief of the security council under Yeltsin.