At least 11 people were killed on Wednesday in one of the worst days of violence since protests against Nicaraguan President Daniel Ortega began more than a month ago, a rights group said on Thursday.

The bloodshed was condemned by the Central American country's Episcopal conference of Catholic bishops, who called it "organized and systematic aggression" and suspended talks with the government scheduled for Thursday.

Witnesses said pro-government armed groups opened fire on the marchers during a demonstration on Wednesday, Nicaragua's Mother's Day. The march was held to remember the children who were among the more than 80 killed since the start of protests more than a month ago.

The Nicaraguan Human Rights Centre said six people were killed in the capital of Managua, with five others killed in other parts of the country and 79 injured. The army said it was treating some of the injured.

A separate count by The Associated Press, based on information from non-governmental organizations and hospital sources, put the death toll in Managua at 13 people.

The European Parliament on Thursday condemned what it called "brutal repression" in Nicaragua and called for elections to be held earlier, echoing other calls for Ortega make the 2021 presidential election earlier.

Demonstrators help an injured man during clashes with riot police at a protest against Nicaragua's President Daniel Ortega's government in Managua on Wednesday. (Oswaldo Rivas/Reuters)

In a letter to Ortega published on Twitter, business association COSEP urged the 72-year-old president to bring forward the next vote at a date to be agreed between the government and representatives of civic society.

"Given the magnitude of this crisis, we urge you to undertake every effort in your power to find a peaceful solution before we find ourselves immersed in an even more tragic situation," the letter said.

The next presidential election is scheduled for 2021.

'Worn out' political model

Carlos Pellas Chamorro, the country's most prominent businessman who is believed to be the Central American country's first billionaire, said in an interview with La Prensa newspaper that the Ortega government's political model is "worn out."

He called for the election of a new government through "a free and transparent process."

Pellas urged the immediate resignation of the entire electoral council, which has been accused of manipulating things to allow Ortega to consolidate power during his last 11 years in office.

Ortega told supporters that Nicaragua "is not private property" in response to the COSEP demand, according to local newspaper La Prensa.

The anti-government protesters were taking part in a march in support of the Mothers of April movement, representing women whose children died in protests. (Diana Ulloa/AFP/Getty Images)

Proposed changes to Nicaragua's social security system last month triggered student-led protests and violent clashes.

Indignation at the crackdown — in which dozens of people have been killed, over 800 injured and more than 400 arrested — has become a daily challenge to the rule of Ortega, a former Marxist guerrilla leader.

Talks sponsored by the Roman Catholic Church on ending the crisis were suspended indefinitely last week due to a lack of progress.

Erika Guevara-Rosas, Americas director for Amnesty International, said on Twitter that Ortega's "violent repression has reached extreme levels" after the attack.

President Ortega´s violent repression has reached extreme levels. A violent attack against the massive peaceful demonstration organized by the mothers of those killed during protests demonstrates the government´s "shoot to kill" strategy. <a href="https://twitter.com/hashtag/SOSNicaragua?src=hash&ref_src=twsrc%5Etfw">#SOSNicaragua</a> <a href="https://t.co/L7eiGgVkru">pic.twitter.com/L7eiGgVkru</a> —@ErikaGuevaraR

A Cold War antagonist of the United States who served a single term as president during the 1980s, Ortega returned to power in 2007. He was re-elected by a landslide for a third consecutive term in 2016 with his wife as vice-president.

The Organization of American States last week called for early elections, but Ortega has not acceded.