Twenty-one people are reported dead and at least 51 injured after bomb blasts once again rocked Bali.
At least three bombs went off in the popular tourist resorts of Kuta and Jimbaran Beach in the south of the Indonesian island as holidaymakers and locals thronged the areas at around 6.50pm local time.
Reports said at least 25 people were killed and more than 100 injured, including many foreigners, when the bombs detonated almost simultaneously at a restaurant in Jimbaran and at the Matahari department store in Kuta.
The attack came almost three years to the day when 202 people- including 26 Britons - were murdered in a terrorist attack by an al Qaida-linked group.
The Foreign Office said there was no reports that Britons were involved but the British Ambassador to Jakarta, Charles Humfrey, was on his way to the affected area.
A spokeswoman said a helpline had been set up, adding: "We are also sending an emergency deployment team from Hong Kong and supplementing this with staff from London.
"There is no word on British casualties." The Foreign Office confirmed that an international rugby team, with some British members, was safe as well as a large volunteer operation, also with some British workers.
The emergency deployment team would be made up of experts who were "used to dealing with crisis situations" whether terrorist attacks or natural disasters, she added.
News of the bombs came as Londoners united against terrorism in defiance of the Tube and bus bombings that rocked the capital on July 7.
Thousands of people gathered in the West End shopping street to attend a pop concert.
It was in Kuta on October 12, 2002 that terrorists associated with al Qaida set off two bombs close to each other that killed 202 people and injured another 209.
The dead - mostly foreign tourists - were killed when bombs ripped through two nightclubs at Kuta beach.
Kuta is a busy tourist area featuring hundreds of hotels, restaurants and bars.
Jimbaran Beach is 10 minutes' drive from the Ngurah Rai international airport and is well known for its five star resorts and beach restaurants.
Major General Police Ansyaad Mbai, a top Indonesian anti-terrorism official, said today's blasts "were clearly the work of terrorists".
Pribadi Sutiono, from the Indonesian Embassy in London, said one of the bombs at Jimbaran beach had been close to the Four Seasons Hotel while the one in Kuta was at the Matahari department store.
Four buildings around the Raja restaurant in Jimbaran had also been completely destroyed, he said.
The Sanglah Hospital, which is the largest of the Bali hospitals, reported 12 people dead and about 40 injured.
Baharat Gorasia and his wife Dipa Hirani, both 27, from north west London, were shopping in the Matahari shopping centre in Kuta until 4.15pm.
The couple, who are staying at the Risata Bali Resort, said they had often dined near the shopping centre over the last 10 days.
A planned trip to see dolphins tomorrow meant they were due to get up at 1am and so did not stay out for the evening.
Mr Gorasia said: "It's very scary. I got a chill down my spine when I heard where the bomb went off - we were in the same area just hours before.
"If we had ended up going in a few more shops we would probably have been caught up in the blast." Mrs Hirani added: "It's shocking. The only reason we didn't stay out later was because we had to get up in the middle of the night." Claire Braden, whose brother Daniel was among the 28 Britons killed in the 2002 Bali bombings, said she was also shocked at news of today's blasts.
Ms Braden, from Brighton, said: "It's just very upsetting to see it happening again and it brings it all back.
"I'm just hoping it is not on the same scale as it was when Daniel was killed." She said members of the UK Bali Bombing Victims
Group were planning a trip to the island for the third anniversary of the blasts on October 12 - but did not know whether the trip would now go ahead.
Sarah Bolam, from Kuoni, said the company currently had 160 clients in Bali. She added: "We have been able to contact the vast majority of these and as far as we know all our passengers, along with our staff are safe and well.
"We are adhering to Foreign Office advice on travel to Bali, which has been updated following the incidents and is not advising against travel to the destination." Holidaymakers who wished to come home would be brought home as soon as possible, she said.
The Foreign Office helpline for friends and family is: 020 7008 8765.