RNA - Speaking in Parliament Square as part of a March Against Racism held in London, Begg referenced a speech by Trump in which he said he would send more inmates to the controversial facility, ITV reported.
According to local media, about 30,000 people took part in the peaceful March Against Racism in London in an expression of unity against a rise in racism, Islamophobia and anti-Semitism, Organizers said the demonstration would also show support for refugee and migrant rights after Brexit and Trump's election.
Amid an atmosphere of "defiance and unity," protesters arrived at Portland Place in Central London holding banners with messages including "Hands off EU workers," "Refugees and migrants welcome here" and "Stand up to Trump," the Independent said.
It was part of a string of demonstrations taking place across Europe to mark the UN's International Anti-Racism Day, with cities including Berlin, Vienna and Athens, as well as UK cities Cardiff and Glasgow, taking part.
Begg told the crowd: "The rise of the far-right and the Nazis and fascists has seen a new wave with the election of Donald Trump, who said when he came to power, 'I'm going to load up Guantanamo with some bad dudes.'
"So my response is: 'When are you going, dude?'"
He joined Tottenham Labour MP David Lammy in criticising a small group of counter-protesters who were suspected of being from the far-right English Defence League.
Begg said that while the influence of the EDL had decreased in recent months, the job to "smash" them was not over, "because some of their views have become mainstream".
Begg spent almost three years in US custody between 2002 and 2005, as he was suspected of terrorist affiliations. He was later released and has spoken widely of his experiences as a prisoner in the Guantanamo Bay institution and other detainment camps under American control.
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