The United States says it has detained a man it claims to be al-Qaeda's "top man" in Asia and the planner of last year's bombing in Bali.
A senior Thai general said the man, known as Hambali, and an unidentified woman were arrested by Thai and US authorities on Tuesday in Ayutthaya, about 50 miles north of Bangkok.
US officials said Hambali, thought to be operations chief of Southeast Asia's militant Jemaah Islamiah (JI) network, was being interrogated. They would not say where.
He is regarded as one of the world's most dangerous following the Bali bombs that killed more than 200 people last year.
But there were also fears that JI might strike again - little more than a week after a suicide car bomber killed 12 people at a luxury Jakarta hotel - in revenge for the capture of the man believed to have planned their operations.
Ms Susanna Miller, spokeswoman of the Bali Bombings Victims Group, said Hambali's arrest was highly significant.
"It is crucial for us that the perpetrators of the Bali bombing are caught and brought to justice. We see this as a very significant arrest and we will be monitoring the developments carefully."
Ms Miller lost her brother Dan in the bombing which tore through a nightclub on the holiday island on October 12th last year and killed 202 people, including 26 Britons.
She said: "Whilst nothing will alleviate the devastating pain and damage afflicted upon so many lives through this tragedy, it is vitally important that this does not happen again. If Hambali is proven to be responsible, we hope that his arrest will make the world a safer place."
The brother of another victim warned Hambali should not be "tucked away" in the US detention centre Camp X-Ray, but tried in an international court.
Mr Tobias Ellwood, who lost his brother Jonathan, said the arrest was the first to provoke an emotional response. But he said stability and peace would not return to the region until the masterminds controlling the terror groups were captured. "This arrest shows that the United States is taking an interest in the area and I would like to see the British and Australians follow suit," he said.