The Home Office has increased its powers to strip jihadists fighting in Syria of their British nationality.
A statement on Sunday night said: "Citizenship is a privilege, not a right, and the Home Secretary will remove British citizenship from individuals where she feels it is conducive to the public good to do so."
Benjamin Ward, deputy director of Human Rights Watch Europe and Central Asia Division, said: "If there is a national security dimension to the stripping of citizenship and if that is something that would be known to the other country of nationality, then that would give rise to concern.
"It's obviously very important that in looking at these issues the Government complies with its human rights obligations."
More top news
The Metropolitan Police Federation, which set up the official JustGiving site, said it felt a "suitable amount" had been collected.
There is no evidence the Westminster attacker Khalid Masood was associated with so-called Islamic State or al Qaeda, the Met Police said.
Janet Ajao said in a statement: "I do not condone his actions nor support the beliefs he held that led to him committing this atrocity."