Russia's top security official said authorities had averted a "large terrorist attack" in the Moscow area by militants armed with homemade bombs and other weapons.
Russian authorities frequently claim to have foiled attacks by militants from the North Caucasus, a volatile region plagued by an Islamist insurgency, but claims to have foiled large-scale assaults targeting Moscow are rare.
"Literally several days ago ... a large terrorist attack was averted at the preparation stage in the Moscow area," Federal Security Service director Alexander Bortnikov told President Dmitry Medvedev in televised comments.
He said four suspects from the mainly Muslim North Caucasus had been detained and that accomplices had been identified.
Mr Medvedev and Prime Minister Vladimir Putin, president from 2000-2008, have struggled to contain the Islamist insurgency, and the Kremlin is nervous about possible attacks ahead of December parliamentary elections and a March presidential vote.
Mr Medvedev and Putin, still seen as Russia's paramount leader, have suggested that one of them will run for president.
Addressing the Kremlin leader, Mr Bortnikov said the suspects had planned to target "crowded facilities and transport infrastructure" and that security officers had confiscated homemade bombs, other weapons and a map with an attack plan.
Insurgent leader Doku Umarov claimed responsibility for a suicide bombing that killed 37 people at Moscow's busiest airport in January and twin bombings on the Moscow metro that killed 40 in March 2010.
Mr Umarov has said he has readied dozens of potential suicide bombers and has threatened more attacks this year.
Speaking as part of a semi-annual report to Mr Medvedev on security forces' fight against militants, Mr Bortnikov said 169 "terrorist" crimes had been recorded in Russia this year, 110 of them in the North Caucasus province of Dagestan.
He said 95 law enforcement and security agents had been killed and more than 200 wounded fighting militants this year.
Mr Medvedev said the number showed that "the situation remains highly, highly tense" but praised security forces for preventing what Bortnikov said were 52 planned attacks in that period.
Separately, a newspaper reported that Russia had began large-scale air force training manoeuvres in the North Caucasus today in a step towards boosting security in the region