Congressman Jamie Raskin, one of the prosecutors, said: “We cannot have presidents inciting and mobilising mob violence against our government and our institutions because they refuse to accept the will of the people.”
Mr Trump’s hastily assembled legal team admitted changing tactics because of the powerful prosecution opening.
Mr Castor’s remarks during the 47-minute opening speech appeared to have persuaded at least one Republican senator to side with the Democrats.
“Trump’s legal team... talked about everything but the issue at hand,” said Senator Bill Cassidy, who voted for the trial to continue.
Trump loyalists also criticised Mr Castor’s defence of the ex-president. “There is no argument. I have no idea why he’s saying what he’s saying,” said lawyer Alan Dershowitz.