A close associate of exiled Russian tycoon Boris Berezovsky is being held over the killing in April of a prominent member of the Russian parliament.
Yushenkov fell out with Berezovsky last autumn
Mikhail Kodanev was a senior official of the Liberal Russia party, which last year split into two factions, one supporting the murdered politician, Sergey Yushenkov, the other supporting Mr Berezovsky.
Interfax news agency quoted Russian Interior Minister Boris Gryzlov as saying that Mr Kodanev and a former aide were taken into custody in the northern Urals town of Kudymkar and sent to Moscow.
Mr Kodanev was said to be expecting his arrest following the detention on Wednesday of two men believed to have carried out the shooting.
The agency quoted Valentina Dubrovskikh, a regional party official, as saying the two men testified that Mr Kodanev was behind the killing.
This arrest was just like something out of 1937
Mr Yushenkov was shot dead outside his home on 17 April, the same day that his faction was registered as the official Liberal Russia party by the Justice Ministry.
The killing stunned Russia's political elite and sparked calls for an end to the climate of impunity that has often surrounded political killings.
There was some speculation that Mr Yushenkov was killed because of his efforts to prove that a series of devastating bomb attacks in 1999 were the work of the Russian secret police.
Several people were detained in the weeks after the killing but all have been released without charge.
Mr Gryzlov said on Wednesday that the murder was not politically motivated but resulted from financial disagreements between the two factions of Liberal Russia.
'Only real opposition'
Yuri Nisnevich, a spokesman for Mr Yushenkov's party, told the BBC that the investigators' theory was "probable".
Berezovsky is fighting extradition in the UK
But Mr Berezovsky, who is currently fighting against an attempt by Russia to extradite him from the UK on charges of corruption, said the affair was reminiscent of Stalin's repression.
"This arrest was just like something out of 1937," he told AFP news agency.
He vowed to continue funding the party.
"I have invested more than $10m in Liberal Russia
because this is the only real opposition party to Putin," he added.
"Kodanev was elected co-chairman of Liberal Russia under my
initiative. He is a key member of our party, a key opposition figure