Shares in online gambling companies slumped after the boss of online gaming group BetonSports was detained while changing planes in the US.
The US arrest was a nasty shock for the online gaming sector
BetonSports shares ended down 25% in London trading while PartyGaming shares finished 6% lower.
David Carruthers, BetonSports chief executive, was travelling to Costa Rica when federal authorities arrested him at Dallas airport.
A spokesman said the firm was trying to found out the reason for his arrest.
At the close of trading, BetonSports shares were down 24.5 pence, or 16.6%, at 122.5p while PartyGaming shares were down 5.5%.
BetonSports is based in Costa Rica and Mr Carruthers, 48, was heading there with his wife Carol when he was arrested late last night.
BetonSports is waiting to hear why its top executive is being held, and so are all of its online betting rivals.
"They've all been on the phone asking about this," spokeswoman Ginny Pulbrook said.
BetonSports, whose shares are listed in London, is based outside of the US to comply with strict gaming laws there.
Mr Carruthers has attacked a proposed US law banning banks and credit card companies from processing internet gambling payments.
Despite the bill winning the backing of the House of Representatives last week, he said the move would fail due to a backlog of US legislation.
Ms Pulbrook said the arrest had come as a complete surprise and that BetonSports had received no indications of any legal problems.
Mr Carruthers and his wife have lived in Costa Rica since 2000.
They had been in London for the company's annual meeting (AGM).
BetonSports is just one of the companies cashing in on an internet betting boom and has 1.2 million customers worldwide, many of them in the US.
In the US online gaming is a $12bn a year business that is expanding despite the government's opinion that it violates a law against placing interstate bets using telephone lines.
This motivated the Congressional action.
Four million people gamble online in the UK every month and the sector is forecast to grow by 22% this year.
However the huge rise in online bets has been blamed for a parallel increase in the number of people seeking help for gambling addiction.