Detainees’ advocates say the statistics point up the flaws in Britain’s approach since Sept. 11. The government says they show how real the threat is.

More than half of those arrested on suspicion of terrorism-related offenses in Britain since Sept. 11, 2001, have been freed without being charged, according to government statistics released Wednesday.

Of the 1,471 people detained, 521 were charged and 196 of those were convicted of terrorism-related charges, the British Home Office survey shows; 819 were released without being charged. Some were charged with offenses not related to terrorism, such as theft, fraud or an expired visa. The report does not say how many of those were convicted.

Advocates for those arrested said the data were evidence of flawed tactics by British security officials.

“We submit this is a very low conviction rate of those arrested for terrorist-related offenses and signifies that the security services are acting on poor or misleading intelligence,” said Mohammed Ayub, an attorney for three Pakistani students recently arrested on suspicion of terrorism and later released. “Continuation of such operations is likely to lead to a lack of public confidence in the security services.”

To the contrary, Policing and Security Minister Vernon Coaker said, the statistics clearly show the “real and serious threat from terrorism” Britain faces.

“The figures outlined today show 196 terrorist-related convictions between September 2001 and March 2008,” he said. “There can be no doubt about the nature and complexity of the threat: aspirations to use a dirty bomb; the targeting of shopping centers, nightclubs and our transport infrastructure; the desire to inflict mass casualties on the public without regard to race, creed or color; the aspiration to commit terrorist acts abroad; and the encouragement and support of terrorism both in the UK and overseas.”

Nearly all terrorism-related suspects arrested in Britain since Sept. 11 are Muslims -- 91%, the report says.

Sixty-two percent of the 125 prisoners serving terrorism-related sentences in England and Wales are Britons, 21% are from Africa, 9% from the Middle East and 4% from Asia.

The largest number of convicts are of Asian ethnic origin, with whites next and then blacks, according to the data.

Home Office chief statistician Paul Wiles told a British newspaper that the figures were probably an “underrecording” of what the final conviction rate will be.

“This is not a complete record of what finally happened to all the arrests in this bulletin,” he said.

But others said the numbers reflected a disturbing trend.

“This astonishingly low number means one of three things: Either the police are arresting the wrong people, or they are failing to bring enough evidence on which a charge can be based, or the evidence produced is insufficient to convince juries,” Pauline Neville-Jones, opposition Conservative Party spokeswoman for security, was quoted as saying by the Guardian newspaper.

“Perceptions that the first of these options is the case -- that the police are victimizing innocent people -- are rife in some parts of Britain. They are not helped by the recent case in the northwest of England in which 12 men were arrested by armed police, only to be released without charge a few days later,” she said.

The main offenses leading to convictions were possession of articles for terrorist purposes, membership in proscribed organizations and fundraising for terrorist purposes. Two prisoners are serving sentences for murder.

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Stobart is a news assistant in The Times’ London Bureau.