Up to two million Uighurs have been rounded up en masse in the past two years and put in detention camps in the northwestern region of Xinjiang for crimes as minor as teaching Islam or downloading messaging apps.

Many have detailed torture and brainwashing in the camps, with recent reports saying mass rape is being committed against Uighur women.

But the Toronto Wolfpack, who Williams has signed with for the 2020 season, said in a statement the team doesn't take a political stance.

"While the Toronto Wolfpack will never take a stance on political issues, our players will always have a right to freedom of speech," the spokesperson said.

In his tweet on Monday, Williams echoed Arsenal playmaker Ozil - also a practising Muslim - that more countries should speak out against China's policy of detaining Uighurs in re-education camps.

"It's a sad time when we choose economic benefits over humanity #Uyghurs," Williams wrote, accompanied by an image illustrating oppression against the Muslim minority group.

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Rugby League: Toronto Wolfpack say Sonny Bill Williams has 'right to freedom of speech' after Uighur tweet - Newshub