Thousands of people chanted, picketed and marched on cities across America as May Day demonstrations raged against President Donald Trump’s immigration policies.

Police in Oakland, California, arrested at least four activists who chained themselves together to block a county building. More than 100 other activists there demanded an end to what they called a collaboration between county law enforcement and federal immigration agents.

Despite the California clash, the initial rounds of nationwide protests were largely peaceful as immigrants, union members and their allies staged a series of strikes, boycotts and marches to draw attention to the importance of immigrants in the United States.

The demonstrations on May Day, celebrated as International Workers’ Day, follow similar actions worldwide in which protesters from the Philippines to Paris demanded better working conditions.

“On this day, we will not go to work,” said Francisca Santiago, a farmworker from Homestead, Florida. “We will not go to school. We will not buy anything.”

In Philadelphia, 1,000 teachers, who’ve been working without a contract for years, protested outside schools.

Supportive parents joined the teachers, many of whom took sick days to protest. Schools were open, and the district said it was working with principals and substitute teachers to make sure classes wouldn’t be disrupted.

In Washington, DC, commercial construction company owner Salvador Zelaya paid his employees to take the day off to attend a march. The Salvadorian business leader said his 18 workers were spending the morning making banners to take to a rally that would end in front of the White House.

Zalaya offered a simple message for the Republican president: “All of us, we are immigrants. We came to this country. We work hard. We build up our own business. We employ people. We pay taxes, and we make America great.”

In a Los Angeles park, several thousand people waved American flags and signs reading “love not hate” as demonstrators prepared to participate in May Day marches expected to be larger than in recent years, buoyed by opposition to the new administration.