UPDATE 1-Russian police raid BP venture, question managers

(Adds BP source with details, colour from scene)

By Anastasia Onegina and Dmitry Zhdannikov

MOSCOW, March 19 (Reuters) - Russian police on Wednesday raided offices of British oil major BP (BP.L) and its venture TNK-BP (TNBPI.RTS) and questioned managers, in a move likely to spark concerns about the Kremlin's new asset-grab campaign.

TNK-BP is the subject of long-running market speculation that the Kremlin wants one of its own companies, such as state gas monopoly Gazprom (GAZP.MM), to buy out the Russian billionaire shareholders in the venture and become a partner of BP, in a bid to further tighten its grip on energy resources.

But the crackdown also comes as Moscow's relations with London are at an all-time low following Russia's repeated refusal to extradite to Britain an ex-KGB agent suspected of murdering a vocal Kremlin critic.

Gazprom's chairman, Dmitry Medvedev, won Russia's presidential election by a landslide earlier this month and has promised to follow the policies of his mentor, Vladimir Putin, the architect of renationalisation of the oil industry of the past year.

The employees of TNK-BP, Russia's No. 3 oil producer, were briefly banned from moving in and out of the firm's headquarters on the famous Arbat street in central Moscow on Wednesday morning after law enforcement officers arrived in two minivans.

"In the evening, the employees have been asked to leave the office earlier," said one company source. A nearby and much smaller office of BP on Novy Arbat, which hosts oil trading and other operations, was also raided in the evening.

"They are collecting information and they are questioning the country president," said a BP source who was not in the building but said he had been in telephone contact with colleagues there.

"They are FSB, they are badged up and they have firearms," he added. "We have the 17th and 18th floors and on one floor they are holding about 20 people and about 15 to 30 people on another floor."

Most staff were later allowed to leave the office, while senior managers were still being held in the building, the source said.

The Russian shareholders in TNK-BP have repeatedly denied that they have plans to sell their 50 percent stake. BP controls the other 50 percent in TNK-BP.

"Everyone will perceive these searches as a start of a large-scale campaign against the shareholders. And I would not disagree with this," said a high-level industry source.

Searches and document seizures became commonplace during a campaign against oil company YUKOS, which was brought to its knees under a multibillion-dollar back tax claim that led to its bankruptcy and asset sales at state-forced auctions.

CONFLICTING STATEMENTS

The interior ministry gave conflicting statements as searches progressed, while TNK-BP declined to comment.