MEXICO CITY — Extraordinary protests against the government of President Daniel Ortega of Nicaragua extended into a third day on Friday as thousands of demonstrators took to the streets in Managua, the capital, and other cities, clashing with government security forces and barricading neighborhoods in opposition to newly announced changes in the social security program.

At least three people, including a police officer, have been killed in the protests since they began on Wednesday, according to the authorities, and dozens of people have been wounded.

The demonstrations have been partly driven by students from the country’s public universities, which historically have been a faithful base of support for Mr. Ortega. They have been joined by a variety of groups, including retirees. The protests are among the largest and most violent in Nicaragua’s recent history.

They were set off by changes to the social security system approved by the Ortega administration this week, but they also tapped growing discontent with the government, protesters and analysts said.