SEATTLE—Democratic presidential candidate Bernie Sanders was shoved aside by several Black Lives Matter activists and eventually left a Saturday afternoon event in Seattle without giving his speech.

Mr. Sanders was just starting to address several thousand people gathered shoulder to shoulder at Westlake Park when two women took over the microphone. Organizers couldn’t persuade the two to wait and agreed to give them a few minutes.

As Mr. Sanders stepped back, the women spoke about Ferguson and the killing of Michael Brown and held a four minute moment of silence.

When the crowd asked the activists to allow Mr. Sanders to speak, one activist called the crowd “white supremacist liberals,” according to event participants.

After waiting about 20 minutes, Mr. Sanders himself was pushed away when he tried to take the microphone back. Instead, he waved goodbye, left the stage with a raised fist salute and waded into the crowd. He shook hands and posed for photos with supporters for about 15 minutes, and then left.