Abu Hamza has been jailed for life in America after being found guilty of 11 terror offences.
The hook-handed cleric, who only has one eye, was convicted of supporting terror organisations including the taking of Western hostages in Yemen, four of which were killed.
He was also found guilty of sending two followers to Oregon to establish a militant training camp and dispatching an associate to Afghanistan to aid al Qaida and the Taliban against the US.
Hamza, 56, was said to have run a "global campaign of terror" from Finsbury Park Mosque in London.
He was extradited from the UK in 2012, 10 years after the US first issued an initial warrant for his arrest.
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The memorial in St James' Park London, surrounded by wreaths and flowers, is wrapped in black bin liners following the incident.
The group trades from about 200 cafes.