WikiLeaks founder Julian Assange, who was arrested on Thursday at the Ecuadorian Embassy, is being held in one of the UK's most notorious prisons.

Belmarsh, built in 1991 and located in southeast London, gained notoriety in the wake of the 9/11 attacks when foreigners were detained in the prison without being formally charged, leading some to call it 'Britain's Guantanamo Bay.'

An unannounced inspection of the jail in early 2018 by the UK government showed progress in some areas, but regression in others.

An action plan developed the prison in the wake of the inspection acknowledged areas where improvements could be made, but pushed back against some suggestions, including not keeping three men in cells meant for two.

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Julian Assange is being held in a notorious UK prison previously referred to as 'Britain's Guantanamo Bay.'

The WikiLeaks founder spent nearly seven years in a 330-square-foot room in the Ecuadorian Embassy in London where he had been seeking political asylum. On Thursday, Ecuador revoked his asylum, allowing British police to arrest him.

As Assange awaits potential extradition to the US, he's been sent to Belmarsh, considered to one of the UK's most high-profile prisons.

Located in southeast London, Belmarsh rose in prominence following the 9/11 terrorist attacks. Under anti-terrorism laws created soon after September 11th, foreign nationals suspected of terrorism could be detained in the prison, according to a BBC News story in 2003. As a result, the prison garnered the nickname 'Britain's Guantanamo Bay,' a nod towards the US detention camp at a military base in Cuba of the same name that gained notoriety for some of its torture tactics.

A report on an unannounced inspection of the prison by the UK's chief inspector of prisons in early 2018 paints a complicated picture. The report acknowledges that over 100 prisoners have an indeterminate sentence, in addition to those who have committed serious offenses.

Read more: US asks to extradite Julian Assange over leaked state secrets after he was arrested and forcibly removed from Ecuador's London embassy

"The high security unit (HSU), in effect a prison within a prison, held some of the highest-risk prisoners in the country, adding a further layer of complexity," the report stated. "In addition, there were a large number of foreign national prisoners, others who needed to be protected because of their offence, and a small number requiring specific management arrangements because of their public and media profile. Meeting the demands and priorities of these various groups remained a hugely complicated task."

While the report determined the prison was generally "well run," it did determine areas of weakness.

The prison failed to meet 37 of the 59 recommendations the report made to the staff following its inspection. And while rehabilitation and release planning had improved since the last visit and safety remained "reasonably good," the prison faltered in other areas. "Respect," a measurement of whether or not "Prisoners are treated with respect for their human dignity" decreased from "reasonably good" to "not sufficiently good" and "purposeful activity" dropped from "not sufficiently good" to "poor."

Violence in the prison also reportedly increased, but Belmarsh noted similar increases in other area prisons at the time.

The 2018 report made 40 recommendations for improvements, of which the prison agreed or partially agreed to attempt to improve upon in its action plan published in June of that year.

Of the recommendations not agreed upon by the prison, one included the need to no longer house three men in cells meant for only two individuals. While the prison acknowledged holding three men in a double cell was not a "desirable practice," it did not breach standards.

"There is no prospect of meeting this recommendation in the medium term," the report stated. "The wider problem of crowding in prisons is a longstanding national issue that can only be addressed through sustained additional investment in the estate over the long term."

Despite the findings of the report that suggest somewhat cramped conditions, individuals on Twitter mocked the comparison of Belmarsh to Guantanamo Bay, noting that the prisoners have access to "education, workshops, two gyms, and a library" along with "therapy and counselling groups," according to a 2006 BBC article.

English actress, writer, and TV personality Emma Kennedy pushed back on the comparison by writing, "We don't have a version of Guantanamo Bay... it was where they sent prisoners detained under the Terrorism Act. Which has now stopped."