Radical Jordanian cleric Abu Qatada and nine other foreign nationals said to pose a threat to the UK's security have been detained, pending deportation.
Abu Qatada arrived in Britain seeking political asylum in 1993
He and at least two others had previously been held at Belmarsh Prison without charge. Their detentions were ordered by the Home Secretary.
It follows an agreement made on Wednesday between the UK and Jordan that deportees would not be persecuted.
Human rights groups say their safety after deportation is not guaranteed.
The detainees are being held at Full Sutton Prison, near York, and Long Lartin Prison, in Worcestershire.
They were arrested in London, Luton, Leicestershire and the West Midlands.
In a separate development the radical cleric Omar Bakri Mohammed has been arrested in Beirut, Lebanon.
Keith Best, chief executive of the Immigration Advisory Service, said the men could have lengthy rights of appeal to the Special Immigration Appeals Commission (Siac).
He said: "The process can drag on for months if not years."
After Siac has made its decision, the detainees can lodge further appeals to the Court of Appeal, the House of Lords and, eventually, to the European Court of Human Rights at Strasbourg.
Home Secretary Charles Clarke announced Thursday's detentions, but did not reveal names.
"The immigration service has today detained 10 foreign nationals who I believe pose a threat to national security," he said.
He added: "The circumstances of our national security have changed. It is vital that we act against those who threaten it."
Gareth Peirce, who represents a number of former Belmarsh detainees who have now been held, said Thursday's arrests were "insane and dangerous government at its worst".
Under the Human Rights Act, the UK cannot deport anyone to a country where they may face persecution.
Some of those arrested come from Lebanon and Algeria, as well as Jordan, all of which have been criticised for poor human rights records.
The government has now reached agreement with Jordan that deportees will not be persecuted, and is in negotiation with 10 other countries, including Lebanon and Algeria.
The Memorandum of Understanding reads: "It is understood that the authorities of the United Kingdom and of Jordan will comply with their human rights obligations under international law regarding a person returned under this arrangement."
RADICAL CLERIC ABU QATADA
Given refugee status in 1994 after arriving in UK on fake passport
Detained in Belmarsh jail in 2002 under laws introduced after 9/11 attacks
Linked to al-Qaeda by a British judge
Released under control order in March 2005
In it, Jordan and the UK undertake to humanely treat any deportee who is detained, quickly bring them in front of a judge and allow them access to lawyers.
Human rights groups including Liberty and Amnesty International believe such agreements will do nothing to safeguard the rights of those who are deported.
Ms Peirce said: "The Home Office did not think it necessary to give a single word of explanation to those individuals as to why this morning they can be safely deported to their respective countries of origin when last night they could not.
"The men themselves in any event have been throughout today deliberately put out of reach of lawyers who represent them. We do not know where they are and the Home Office will not tell us."
Abu Qatada was one of the Belmarsh detainees, held in the high security jail without charge for about two years.
He has been sentenced in his absence to life in prison by a Jordanian court in relation to a series of explosions there.
At least two other former Belmarsh detainees, known only by the letters Q and I, are said to be among those arrested on Thursday.
The man known as I is an Algerian who claimed asylum in the UK in early 1995. He was detained in April 2002, accused of supporting and raising funds for terrorist groups.
They were all released under "control orders" - a type of house arrest - earlier this year.