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The blast killed 12 people - including children - and injured 30 others and was the second worst since the present wave of troubles began in 1969.
Only six of the bodies have been formally identified after the petrol bomb exploded in the La Mon House entertainment complex at 2100GMT, in a usually peaceful area of County Down, 14 miles from Belfast. None of the people detained after police and army raids has been officially named and the round-up operation continues.
A police statement described the people they picked up as "suspected of being active in IRA terrorist activities and their numbers include members believed to be prominent in the command structure of the IRA."
The IRA has not admitted involvement in the bombing, but a spokesman for its political wing Sinn Fein revealed nine of their members had been arrested, including executive member Gerry Adams.
Remnants of the bomb timer and two petrol cans have been recovered and forensic experts suggest it is a new kind of device involving a small explosion that throws out a massive flame.
The bomb had been taped to the window of the Peacock Room, where all the casualties were found.
The victims were all Protestants and included 45 members of the Irish Collie Club and the Northern Ireland Junior Motor Cycle Club.
A total of 450 people were forced to flee from elsewhere in the highly flammable plastic and wooden hotel structure.
Northern Ireland Secretary Roy Mason - who flew back to the UK hours before the explosion - said it was "an act of criminal irresponsibility" carried out by "remnants of IRA gangs".
Before leaving Mr Mason made a speech to an Alliance Party delegation about the decreasing terrorist activities of the Provisional IRA.
Many Loyalist activists and politicians blame his complacent attitude for the attack.