Following a day of mourning across Europe and America on Friday, the search effort is continuing in the ruins of the World Trade Center in New York under difficult circumstances.
Thousands attended interdenominational services in Belfast, Londonderry, Armagh and Portadown and a three-minute silence was held on Friday morning, when life in other major European cities also paused.
In Dublin, thousands more attended a memorial service with Irish President Mary McAleese and Taoiseach Bertie Ahern.
Queues of people over a mile long waited to sign books of condolence in Dublin and similar books have been opened by churches and local authorities throughout Northern Ireland.
Services to pray for the dead and their relatives will also be held on Saturday, including one at St Anne's Cathedral in Belfast.
The choir from St Anne's were in New York in July and visited the twin towers of the World Trdae Center.
President visits Manhattan
On Friday President Bush went to see the devastation in Manhattan, on the third day of searching for any survivors and bodies after the double strike on the twin towers using hijacked passenger jets.
He flew over the scene of the disaster by helicopter, before touching down.
Mr Bush has authorised the call-up of about 50,000 reserve troops, in response to the New York and Washington attacks.
Troops are helping in the massive operation to move rubble from the unstable site in downtown Manhattan.
Air force reservists will also patrol air corridors over major US cities.
On Friday thunderstorms and driving rain were hampering the rescue effort in New York.
The list of people still missing in New York is remains at about 4,700. Only about 100 bodies have been recovered.
But it is believed many more were killed.
The US government has now said the number of people killed in the attack on the Pentagon in Washington may be more than 200.
The British Government has said the number of Britons confirmed dead is about 100 and it is feared more than 500 were killed.
The Irish Department of Foreign Affairs has so far confirmed four Irish people were killed in the attacks.
But the Dublin government expects that figure to reach double figures.
The Foreign Office emergency number for British people worried about friends and relatives is: 020 7008 0000.
The Dublin Department of Foreign Affairs emergency number is: 00 353 1 408 2000.