MOSCOW, June 26 (RIA Novosti) - At least 16 supporters of jailed Yukos founder Mikhail Khodorkovsky were detained in downtown Moscow on Friday after they gathered to celebrate the businessman's birthday and protest his imprisonment, police said.

The activists were arrested not far from Red Square, a police spokesman said. He said they had failed to obtain prior permission for the demonstration.

"Some 20 or more people marched ignoring police's demands to disperse. Then police detained the [majority of] activists," the spokesman said, adding they would be released once their identities had been established.

Khodorkovsky, who is serving an eight-year sentence for fraud and tax evasion believed by critics to have been orchestrated by the Kremlin, turns 46 today.

Khodorkovsky's supporters have been fined for "public order offenses" during similar events in previous years.

In the Siberian city of Chita, where the businessman is imprisoned, supporters unfolded a banner with congratulations and set off fireworks outside the jail last year.

The businessman is currently facing a new trial that could see his sentence extended by a further 20 years.