Russia stepped up security in major cities Wednesday, deploying thousands of extra police and urging public vigilance a day after officials warned of a possible terrorist threat on public transportation.

In Moscow, which was last hit by terrorist attacks in 2004, officials took the unusual step of ordering mobile phone service shut off in the subway system in what appeared to be an effort to avert the possibility of explosives being detonated by cell phone.

Uniformed police and bomb-sniffing dogs could be seen in subway stations, train stations and other sites around Moscow, checking documents and standing guard at entrances. Public announcements asked passengers to be alert to suspicious items.

Moscow's three main international airports introduced stricter passenger checks and additional police patrols of air terminal buildings, news agencies reported.

Russian television showed a heightened police presence at other towns around the country.

A top police official, Interior Ministry Col. Gen. Nikolai Rogozhkin, was quoted by RIA-Novosti as saying that 5,000 extra police officers had been dispatched to protect public transport in Moscow, St. Petersburg and other major centers.

Moscow transport police Col. Viktor Ivashchenko said the city had bolstered the number of officers on patrol to 600 and pointed out that some preventive measures being taken by police would not be observable.

"To be honest, I was more cautious than usually today," one Moscow resident, who gave his name as Anvar, told Associated Press Television News. "I was even suspicious when a woman in the Metro placed her bag down there. I'm on guard whenever possible."

Police officers checked the tickets of all passengers who entered the Kiev railway station in central Moscow as security guards with walkie-talkies patrolled the station.

But Sergei Zaitsev, who was waiting for a train at the Kiev station, was skeptical about the security measures.

"All they did was look at my ticket, not my bag. If I wanted to, I could easily take a bomb and plant it on a train," he said. "Terrorism is still a very real threat here."

Tuesday's announcement came amid a lull in terrorist scares and attacks in Russia.

The country saw spate of high-profile terrorist incidents in recent years, including hostage takings and subway and airline bombings linked to the 12-year conflict in mostly Muslim Chechnya, where large-scale fighting ended years ago but an insurgency continues.

In February 2004, an explosion ripped through a Moscow subway car during rush hour, killing 41 people. In August 2004, a suicide-bombing just outside a Moscow subway station killed 10 people.

Also in August 2004, suicide bombers who boarded their planes at a Moscow airport blew up two Russian passenger jets that exploded almost simultaneously, killing all 90 people on board.

The Moscow subway, one of the world's busiest, carries about 8 million passengers a day. After the 2004 attacks, it began installing video surveillance cameras on all subway cars.

On Tuesday, the Federal Security Service that the national anti-terrorism headquarters had received information "from foreign partners ... about the possibility a subversive terrorist act could be committed on ground transport and in the subway."

It was unclear what country the information about the potential threat came from; Russia cooperates against terrorism with countries around the world, including the United States.

The statement did not say whether the threat was limited to any particular location in Russia, where several cities have subway systems.

The Kremlin has scored a series of notable successes in its fight against terrorism, including the death in July of Chechen warlord Shamil Basayev, who claimed responsibility for some of the nation's bloodiest terrorist attacks.

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