Mr Bukovsky says the current secret service is a continuator of the KGB

Mr Bukovsky is back in Russia for the first time since Mr Putin was elected in 2000 - and only the third time since being exiled in 1976.

Mr Bukovsky said he wanted to use his experience gained confronting the Soviet secret police, the KGB.

He was arrested and imprisoned several times for opposing the Soviet system.

"We are seeing a return of Soviet times," Mr Bukovsky told Ekho Moskvy radio hours after his arrival on Tuesday.

He has announced his intention of running for the presidency in 2008 - though he admits he would probably be barred from standing, for failing the 10-year residency clause.

Putin critic

Mr Bukovsky, 64, was arrested, put under surveillance, forcibly treated in psychiatric clinics and imprisoned several times from 1963 until his expulsion from the USSR.

Vladimir Bukovsky is one of Mr Putin's harshest critics

He is a neurophysiologist with degrees from Britain's Cambridge University and Stanford University in California, and the author of several books.

Mr Bukovsky, who lives in Britain, returned to Russia in 1991 for the first time and has been a constant critic of Mr Putin.

Commenting on the murder in London of former Russian agent Alexander Litvinenko, Mr Bukovsky said both Mr Litvinenko and Russian journalist Anna Politkovskaya were killed on orders of the Kremlin.

He said a recent law adopted by parliament in July allowed the president "to use his special services as death squads to eliminate the enemies including [those] abroad".