The suspects were arrested during dawn raids by at least 30 officers, some of them armed, on two addresses in west London.
This follows the discovery by Irish police in Donegal of a suspected IRA arms dump and training ground. They seized two Kalashnikov AK47 rifles, ammunition, a primed "horizontal mortar" or rocket grenade and training manuals.
Five Londonderry men were brought to Dublin last night to face arms charges in the anti-terrorist Special Criminal Court following the discovery on Sunday on a farm in Donegal's remote north-western Inishowen peninsula.
Gardai believe the mortar was being prepared for a possible attack in Northern Ireland.
Two explosive devices wrapped in plastic were found in searches of the surrounding area yesterday.
The raid follows the discovery of a quantity of Mark 6 IRA mortars in the Irish Republic in a raid on a farm at Hackballscross, Co Louth, three weeks ago.
The operation in London was carried out by the Metropolitan Police's anti-terrorism section and Special Branch. The operation is not, at this stage, believe to be linked with any of the IRA recent attacks, such as the Docklands bomb. No shots were fired and no one was injured.
A dozen mortar bombs were fired at Heathrow airport in three separate attacks within days of each other in March 1994.
None exploded, but one landed on the roof of Terminal Four, which was crowded with thousands of passengers, while others narrowly missed parked aircraft. In the first attack, four mortar bombs launched from a car parked at the Excelsior Hotel landed near the north runway.
In the second attack less than 30 hours later, another four bombs were launched, this time from a small copse. They landed just short of the south runway near Terminal Four.
In their trajectory of about 1,000 yards, they passed less than 40 yards from a parked Boeing 747.
In the third attack, a further four mortar bombs were fired and three hit the tarmac about 50 yards from 15 waiting aircraft. The final bomb landed on the roof of Terminal Four.
A neighbour of one of the suspects, who lived in Warwick Road, Earl's Court, described yesterday's 4.30am raid.
Kenji Mitsuhashi, 26, a Japanese student, said: "I looked out of the window - there were six or seven men. They went in next door with big guns and one had a pistol.
"I heard a shout. I saw one man being brought out with hands cuffed behind his back. He was dressed in a white suit, with blond hair in a long, bobbed style. He was about 26 or 27. They put the man who was handcuffed into a car."
The second man was arrested at a nearby flat in West Cromwell Road.Reuse content