The Taoiseach has condemned the attacks in London today as "a black mark on society" and "wanton violence".
Speaking after a private audience with Pope Benedict XVI at the Vatican, Mr Ahern said such acts of terrorism would not affect decisions by the G8 leaders.
"This is terrorism and violence perpetrated against ordinary people . . . it's just a black mark on society, a devastating blow against people," he said.
"This is a huge emergency. A terrible, sad day," he said. "What satisfaction do people get out of that? Will this mean that the G8 leaders will make a different decision than they would have made yesterday? It won't," Mr Ahern said.
"This is all wanton violence. What does this all mean, killing innocent people who were probably on their holidays, going shopping around London for the day," he said.
Mr Ahern was told about the explosions as he was leaving the Vatican after his half-hour audience with Pope Benedict at 11am Irish time. That was followed by an hour-long meeting with Vatican secretary of state Cardinal Angelo Sodano.
Minister for Foreign Affairs Dermot Ahern expressed his "most profound sorrow and concern at the blasts that have devastated public transport in London and resulted in the loss of life and serious injury".
He said: "While we are not aware of Irish casualties at this point, the Department in collaboration with the Irish Embassy in London is closely monitoring the situation."
Speaking in Belfast, Mr Ahern said the Irish embassy in London has been receiving an increasing number of calls from Ireland from people worried about relatives. Mr Ahern said it was not clear if there were Irish casualties.
The Department of Foreign Affairs has set up emergency numbers for those concerned about relatives or friends following the explosions in London today.
The emergency section at the Irish embassy in London is taking calls from UK residents, and a freephone number has been set up at the Department of Foreign Affairs in Dublin.
Irish residents may phone 1800 242548for information. In Britain, the contact numbers are 0207 2012508or 0207 2012501.
Fine Gael leader Enda Kenny sent a message of condolence to the British prime minister Tony Blair following the attacks.
"This morning in London innocent people were viciously targeted by what appears to be another appalling terrorist attack. The timing of this outrage is clearly designed to undermine and distract from the efforts of the leaders of the G8 as they focus on tackling the issue of global poverty.
"At a time when the peoples of the world were uniting to face one of the greatest challenges to our planet, these attacks are a direct challenge to that emerging unity of approach. They must not succeed in their goal," he said.
The Labour Party leader Pat Rabbitte condemned the attacks as "despicable".
"These attacks will repel and horrify anyone with any shred of decency or humanity. With so many Irish people resident in London, there has always been a close affinity between the Irish people and the British capital and all our thoughts will be with those who have been injured or who have lost relatives in these attacks," he said.
Sinn Fein president Gerry Adams said: “I have sent a message of sympathy and solidarity to Mr Blair and the London Mayor Ken Livingstone.
“On behalf of Sinn Fein I offer my sincere condolences to the victims and the families of those killed and injured and to the people of London.”
Green Party leader Trevor Sargent said he was "deeply shocked at today’s events", and he extended his sympathies to the victims and their families.
He said he had contacted the British ambassador here on behalf of his party to express solidarity with the victims and all those bereaved, injured and traumatised by the "unpalatable cruelty of these explosions".
Archbishop Sean Brady, Primate of All-Ireland, said his thoughts and prayers were with the dead, the wounded and their families.
“On behalf of my brother bishops and myself I offer my condolences and prayers to all those affected. I wish to commend the emergency services for their work in responding to this atrocity,” he said.
Archbishop Brady said the sympathy and the support of the entire Catholic community in Ireland was with those affected at this difficult time.
Dr Diarmuid Martin, the Catholic Archbishop of Dublin, said those killed and injured and their families would be remembered as Masses in the Dublin Dioceses tomorrow and over the weekend.
The Archbishop, who was in the London area when news of the explosions emerged, said his thoughts and prayers were with the victims and their families.
The Irish Red Cross said it had offered its assistance to the British Red Cross to deal with the casualties in London.
David Andrews, chairman of the Irish Red Cross, extended his condolences to the injured and bereaved and said the Irish Red Cross was ready to send expert personnel to London if required.