MEXICO CITY—Nicaragua’s embattled President Daniel Ortega moved to tighten his grip on power, as paramilitary forces loyal to his government attacked antigovernment protesters at several key points over the weekend, killing an estimated 11 people.

Three students were killed after clashes at Managua’s main university campus. Meanwhile, as many as eight people died on Sunday as masked gunmen attacked opposition barricades in two provinces, according to the Nicaragua Human Rights Association, a local human-rights group that has...