A radical Islamic cleric who has been accused of being a leading figure in al-Qaeda was a risk to UK security, a deportation hearing has been told.
Abu Qatada did not attend the London hearing
Abu Qatada is trying to persuade the Special Immigration Appeals Commission that he should not be sent to Jordan.
It is the first test of a government agreement with another country meant to guarantee a deportee's safety.
His lawyer said the case relied on US evidence obtained using "torture" which should be ruled inadmissible.
A man held in Afghanistan was "repeatedly subjected to torture and ill-treatment by the US authorities with a view to forcing him to provide details against the appellant," Edward Fitzgerald QC told the London hearing.
Abu Qatada, who did not attend the hearing, has previously denied he was connected to al-Qaeda.
Ian Burnett QC, for the home secretary said 44-year-old Abu Qatada was a "danger to national security".
"Since at least 1995, the appellant has given encouragement to the commission, preparation and instigation of terrorism overseas by providing spiritual and religious advice," Mr Burnett claimed.
"The Security Service assesses that the appellant constructed a support base within the United Kingdom for terrorism-related activities abroad and in the UK." The UK has signed an agreement with Jordan so that anyone sent back will not be tortured or killed.
Abu Qatada's lawyer is expected to argue that the guarantee is inadequate.
Mr Burnett said the so-called memorandums of understanding were not "strictly speaking, a document which is legally binding in international law".
But he said "personal, political and diplomatic commitment" has been made in the negotiations to secure the agreement with Jordan.
There were not "substantial grounds" for believing Abu Qatada would be at "real risk of proscribed ill-treatment" on return.
Abu Qatada came to the UK with his family as a refugee in the mid-1990s. He was originally detained under anti-terrorist measures in 2002.
After the Law Lords ruled that detention unlawful, he was released and put under a control order.
He was then re-arrested, with a view to deportation to Jordan, where he has been convicted of terrorist offences.
In December, Abu Qatada made a video appeal to the kidnappers of British peace activist Norman Kember in Iraq.
The recording, made inside Full Sutton jail, near York, where he was being held pending the deportation decision, was broadcast in the Middle East.
Mr Kember and two other hostages were eventually freed in a planned operation by multi-national forces in March.
Mr Burnett said the government believed Abu Qatada had "associated, or sought to associate, with known Islamic extremists" after he was released from prison on the control order in March 2005.
He alleged: "The appellant has links with the Egyptian Islamic Jihad, now assessed to be part of the al-Qaeda network, through Ayman Al-Zawahiri its sometime leader - who then became, in effect, Osama Bin Laden's number two."
He also claimed Abu Qatada was a "close associate" of Algerian Islamic extremist Abu Doha and the "spiritual leader" to the Al-Tawhid movement, headed by Abu Musab Al-Zarqawi, "who has gone on to international fame as a result of his murderous activities in Iraq".
The hearing on Abu Qatada's possible deportation is expected to last for several days.