Dozens of angry supporters of President Donald Trump converged on vote-counting centers in Detroit and Phoenix as the returns went against him Wednesday in the two key states, while thousands of anti-Trump protesters demanding a complete tally of the ballots in the still-undecided election took to the streets in cities across the U.S.

"Stop the count!" the Trump supporters chanted in Detroit. "Stop the steal!" they said in Phoenix.

The protests came as the president insisted without evidence that there were major problems with the voting and the ballot counting, especially with mail-in votes,...