Amnesty says thousands arrested even killed in "uncertain circumstances".
Human rights abuses in Saudi Arabia have soared as a result of counter-terrorism measures introduced since the 2001 attacks in the United States, Amnesty International said on Wednesday.
The human rights organisation warned in a new report that under the guise of national security, thousands of people had been arrested and detained in virtual secrecy and others had been killed in "uncertain circumstances".
There have long been human rights problems in the kingdom but Amnesty said the number of people being held arbitrarily, including both Saudi nationals and foreigners, "has risen from hundreds to thousands since 2001".
"These unjust anti-terrorism measures have made an already dire human rights situation worse," said Malcolm Smart, director of Amnesty's Middle East and North Africa programme.
Amnesty noted that in June 2007, the Saudi interior ministry reported that 9,000 security suspects had been detained between 2003 and 2007 and that 3,106 of these were still being held.
Some of those held are prisoners of conscience, targeted for their criticism of government policies, the report said.
The majority are suspected of supporting Islamist groups that are opposed to Saudi Arabia's close links to the United States and have carried out a number of attacks targeting Westerners and others.
Amnesty said trials of people suspected of terrorism offences are carried out in secret, despite sentences ranging from fines to the death penalty. The names of those involved or the charges against them are not disclosed.
"Detainees are held with no idea of what is going to happen to them," Smart said. "Most are held incommunicado for years without trial, and are denied access to lawyers and the courts to challenge the legality of their detention."
The Saudi authorities were not immediately available for comment, but the country's top human rights official told AFP last month that suspected militants being tried in special courts were allowed lawyers to help their defence.
"They can choose a lawyer... or the ministry of justice will provide one," said Bandar al-Aiban, president of the official Saudi Human Rights Commission.
He said he regretted that the trials were being kept secret but said the government was worried some defendants would use a public trial as a soapbox to preach radical ideology. "We have to be mindful of other dangers," he said.
Amnesty accused the international community of failing to hold the Saudi government to account over the alleged violations, saying the kingdom "has used its powerful international clout to get away with it".
The group also reported that many people were thought to have been tortured "in order to extract confessions or as punishment after conviction".
Methods include severe beatings by sticks, suspension from the ceiling and the use of electric shocks and sleep deprivation, while "flogging is also imposed as a legal punishment by itself or in addition to imprisonment".
Democracy is key to the rule of law and is a safeguard to Human Rights. Monarachies and Autocratic regimes will be a major disaster for the future generations. I hope the Saudi Government is in the process of Transitioning to a better Government and protection of Human Rights. I doubt it, it doesn't hurt to be Optimistic.