MANAGUA - Thousands of pro- and anti-government protesters took to the streets of Nicaragua's capital Saturday, protesting and backing President Daniel Ortega's bid to remain in power.

At least 12 people were injured -- three policemen and nine protesters -- and some vehicles were damaged, according to political leaders and media.

Chanting "Democracy yes, dictatorship no!" mainly peaceful Ortega opponents numbered at 50,000 by organizers marched in Managua against his controversial reelection plans, even as thousands of the president's supporters gathered for their own demonstration.

Most of the injuries occurred when Ortega opponents, walking on the roads out of the city after their rally, clashed with government supporters who were converging for their own march, the political leaders and local reports said.

Security was heavy in the capital following provocative gestures earlier in the week by both sides which suggested there could be clashes between the two demonstrating groups when they march in the city.

Addressing pro-government marchers, Ortega made an appeal for calm, telling his opponents: "We don't expect you to say that you love us, but simply use your head... instability and confrontation is not the way to go."

Many in the opposition event waved blue and white Nicaragua national flags. They included supporters of opposition political parties, thousands of whom flooded into the city from the countryside to protest, as well as business groups and representatives of the powerful Catholic church.

"We do not want violence," read one banner held up during the six-kilometer (four-mile) march which culminated at the offices of the country's Supreme Court.

Among the demonstrators were former president Arnoldo Aleman and Dora Arguello, daughter of the late former world boxing champion Alexis Arguello.

"This is a triumph for democracy," said Violeta Granera, director of the civil society group Movement for Nicaragua. She accused Ortega of "abuse of power" and of wanting to "dismantle the fragile (public) institutions."

The two marches originally had been called for the same time on nearly the same route, but the risk of violence prompted the pro-government rally to be moved to the afternoon.

More than 7,000 police, including heavily armed riot police, were deployed in the capital to ensure order, an AFP journalist learned.

The opposition demonstration concluded without serious incident in the city's southeast, but Managua remained tense as several thousand Sandinistas gathered to show their support for Ortega.

At least one homemade mortar was fired into the air at the Sandinista march, and several young men were seen marching with bandanas covering their faces.

The government said it hoped to bring some 300,000 people out into the streets to outshine the opposition march, but local media reported far smaller numbers.

Thousands were seen filing into the city, however, to lend support to Ortega and his bid for more victories against the conservatives, which were voted out of power three years ago.

Ortega led the 1979 Sandinista uprising that ousted the regime of US-backed dictator Anastasio Somoza, after 45 years of oppressive rule.

The leftist Ortega, who served as president from 1985-1990, was elected to office again in 2006.

Tension has been building since the ruling Sandinista party's crushing win in mayoral elections a year ago, which the opposition charged were riddled with fraud, and a Supreme Court ruling last month that cleared the way for Ortega to seek reelection in 2011.

Ortega at the rally defended his right to reelection saying, "only the people can decide with their vote" who will be the next president. He warned that the Sandinista party "will not let itself be shackled ever again."

Securing the ability to seek reelection has been a key strategy for leftist regional allies Hugo Chavez in Venezuela, Rafael Correa in Ecuador, Evo Morales in Bolivia, among others. Elected President Manuel Zelaya in neighboring Honduras was thought to have been developing such plans when an army-backed coup toppled him in June.

Saturday he marked two months holed up in Brazil's embassy in Tegucigalpa, where Zelaya has been awaiting reinstatement.