The government said at least 28 police officers have been wounded in the violence, but has not released numbers of protesters hurt or arrested.
Protesters in the small Central American country oppose reforms to the nation's pension system, under which workers and employers would have to pay in more.
Two protesters and a policeman were killed in the capital, Managua, after demonstrations turned violent.
"We are against these reforms, which means we're against this government taking from the pockets of Nicaraguans", said Juan Bautista.
Nicaraguans have been protesting for three straight days against government changes to social security, signed into law earlier this week, that increase worker contributions and lower pensions.
The demonstrations come after years in which the opposition claimed electoral fraud and other malfeasance by the government, but without street protests reaching such magnitude or intensity.
A woman nearby shouted: "The people are exhausted of this repression!" Other students took refuge in nearby buildings or residences.
Four independent television outlets were taken off the air after they broadcast the demonstrations on Thursday, and two were still in the dark Friday.
Vice-President and First Lady Rosario Murillo compared the protesters to "vampires demanding blood to feed their political agenda".
Police did not immediately confirm the deaths, but images of the two young men were carried in local media and repeated on social networks.
A spokeswoman for the UN's High Commissioner for Human Rights, Liz Throssell, urged the government to prevent further attacks on demonstrators and the media. Some demonstrations began peacefully, with protesters waving placards and chanting, "Viva Nicaragua libre!"
In a Facebook post, Channel 15 director Miguel Mora called the action by President Daniel Ortega's government a "clear violation of freedom of the press".