DETROIT — Police on Thursday arrested at least 11 protesters who tried to block buses from picking up Detroit students, the fourth day of demonstrations against voluntary summer classes during the coronavirus outbreak.

The Detroit school district this week began offering online or in-person instruction to students. Students and teachers must wear masks, and class sizes are smaller to reduce virus risk.

But a group of people has appeared each day to protest the program, saying officials are putting people at risk. A lawsuit has also been filed.

"We are out here to prevent the spread of a deadly disease in our schools, to prevent that from spreading from the schools back into the community," said teacher and protester Benjamin Royal.

Eleven people were repeatedly warned before they were arrested for blocking operations at the bus company, police Commander Arnold Williams said.

There were 630 students in classrooms Wednesday and 1,100 more participating online, Superintendent Nikolai Vitti said. Most students don't ride buses to get to one of the two dozen schools.

"We are staying focused on what matters: serving children and their families and adjusting to the new normal with COVID," Vitti said on Twitter.