The Prime Minister John Howard joined Australians in prayer yesterday as hundreds braved the wind and rain to gather at St Andrew's Cathedral for a special service in the wake of London's terror bombings.

Consular officials from several Commonwealth nations joined the PM, including representatives from Britain, New Zealand and Canada as well as the United States.

Other dignitaries included the NSW Deputy Premier Dr Andrew Refshauge and representatives from the Governor's office, the Police Commissioner's office and the Australian Defence Force.

The service was announced the day after four bombs were detonated on London's transport system, killing at least 50 people and wounding more than 700.

Islamic extremists with links to Al Qaeda are being blamed for the series of coordinated attacks across the centre of the British capital.

Mr Howard, who this week urged Australians to rally behind the British people, read from Ecclesiastes 3:1-11.

The US Consul General Stephen Smith gave a reading from 1 Timothy 2:1-8.

The Dean of Sydney Phillip Jensen said the attacks were "evil and wicked' and brought back memories of September 11.

"The death and destruction was not as big as September 11, or Bali, or even the tsunami, but what horrifies us is that it's just so evil because it is an attack on innocent lives and on the freedom of living," Mr Jensen told the congregation.

"It was an act sponsored by evil and wickedness."

The Dean urged prayer for the perpetrators and the victims as well as for the people of London.

"We should pray for our enemies," Mr Jensen said.

"This is unnatural and can be hard psychologically, but Jesus died for his enemies.

"Pray that their hearts may be changed and that this war will cease. Pray that we may love them even as we oppose them."

Mr Jensen said the close ties that many Australians have with Londoners contributed to the shock and anger in the community.

"I love London. It emotionally and symbolically stands for so much of what we hold dear," Mr Jensen said.

"[The attacks have] touched our community consciousness, not only because there are some Australians hurt, but because we feel connected to them."

God Save the Queen was sung as well as Advance Australia Fair at the service as a mark of respect for Australia's British heritage, the Dean said.