Commenting on the investigation into possible links between his campaign and Moscow, the President insisted he was the victim of a “witch hunt”.
On Wednesday, former FBI chief Robert Mueller was named as the man to oversee the probe into alleged Russian interference in the 2016 US election.
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But Mr Trump said the appointment of a special prosecutor to probe his campaign's ties with Russia “hurts our country terribly”.
He claimed it was evidence the US was “a divided, mixed-up, not unified country" and is "a very, very negative thing”.
Mr Trump said in a speech earlier on Wednesday that no politician in history "has been treated worse or more unfairly”.
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Donald Trump: No politician has been treated worse or more unfairly
He was responding to mounting criticism over the surprise sacking of former FBI chief James Comey, who was dispatched from his role at the bureau as it conducts an investigation into Russian involvement in last November’s vote.
Addressing that controversy today, Mr Trump suggested Mr Comey had been fired because he was “unpopular”.
"Director Comey was very unpopular with most people," he said.
"Look at all the people on the Democratic side, not only the Republican side, who were saying such terrible things about Director Comey.”
Initial statements from the White House said Mr Trump had fired Mr Comey on the recommendation of the Justice Department because of his handling of the FBI investigation into Hillary Clinton's private email server.
The President later claimed he had fired Mr Comey after deciding the FBI investigation into his campaign's possible ties to Russia was baseless.
The US President was speaking during a joint press conference with his Colombian counterpart Juan Manuel Santos.