Russia's president has signed a new law that will boost the powers of Russia's security services, in a move that critics say could be used to stifle protests and freedom of speech.

The law, signed by Dmitry Medvedev on Thursday, restores a Soviet-era practice that allows the Federal Security Service (FSB) to issue warnings or detain people suspected of preparing to commit crimes.

Rights groups say the bill raises doubts about Medvedev's commitment to promoting civil rights, and puts the agency, which was formed from the remains of the Soviet KGB, above the law.

Opposition groups also say the law is empowering an already extremely powerful FSB.

Medvedev has defended the law, saying earlier this month that its aim was to improve Russian legislation and had been drawn up on his personal orders.

"Every country has a right to fine-tune its legislation, including in respect to special services," he said.

The Kremlin has also said the law will help combat extremism and prevent illegal participation in anti-government activities such as protest rallies.

Under the 2000-2008 presidency of former KGB agent Vladimir Putin, the FSB dramatically increased its influence over Russian society.