Detectives have tracked down an elusive gang of international computer hackers who are accused of extorting huge sums from online betting sites.

They are suspected of running a global protection racket in which they threatened to crash bookmakers' sites unless payments were made.

Three ringleaders of the Russian gang who made hundreds of thousands of pounds were arrested in a series of raids in St Petersburg, Saratov and Stavropol on Tuesday.

They were held after 10 other members of the gang were found in Riga, Latvia, last November.

Investigators from Britain's National Hi-Tech Crime Unit then followed an electronic money trail to the three gangsters controlling the operation in Russia.

They worked closely with Russian computer crime experts and authorities in the central Asian republics, the Baltic states, Australia, Canada, Estonia and the United States as they tried to locate the ringleaders.

The hackers worked by launching so-called "denial of service" attacks.

Targeted websites are effectively shut down by flooding the server with requests for information.

As many as hundreds of computers could be commandeered through hacking and ordered to send messages.

Online betting services around the world lost millions of pounds in business.

After launching an attack the gang would then make threats by email, demanding money to stop targeting a particular site for a year.

Some victims were unwilling to report the threats to police for fear of negative publicity.

The NHTCU began investigating extortion threats against British sites last autumn.

Since the first threats were reported bookmakers have feared their sites would be paralysed ahead of major sporting events, including this year's Grand National and Euro 2004.

At the start of the year hackers threatened offshore websites used by United States gamblers before the Super Bowl.

It was not clear whether the gangsters arrested in Russia were involved in that sting.

The Irish bookmaker Paddy Power was also hit by an electronic attack on the night of the Super Bowl, paralysing its website for several hours.

A spokesman for William Hill, which suffered a denial of service attack in March this year, said the arrests in Russia were a "huge sigh of relief for all UK bookmakers".

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