Unable to calm student-led protests that just kept growing, Serbia’s strongman leader, Aleksandar Vucic, reached for some tried-and-tested scapegoats, unleashing his media attack dogs on foreign-financed groups that have nettled him for years.

But what started as a familiar ritual of intimidation against groups that document issues like corruption, human rights abuses and electoral fraud — and which Mr. Vucic blames for the protests — has recently taken an unusual and menacing turn.

Encouraged by the Trump administration’s assault on the American agency U.S.A.I.D., Serbian authorities on Feb. 25 sent dozens of police officers, many of them armed, to raid the offices of four nongovernmental organizations. They made the raids without warrants.

One organization, Centre for Research, Transparency and Accountability, said it was raided and that officers carted away copies of 8,500 pages of documents. Another target, Civic Initiatives, said it surrendered 1,300 pages and thumb drives containing confidential personal information about its staff and finances.

Both had received a small part of their financing from U.S.A.I.D., as did a third group. The fourth group did not receive aid from the U.S. agency.