Riot police blocked protesters as thousands of people took to the streets Saturday with tensions rising between opposition leader Juan Guaido and President Nicolas Maduro after crisis-wracked Venezuela emerged from the chaos of an electricity blackout.

Both Guaido and Maduro, who are locked in a bitter power struggle for the right to lead the oil-rich South American nation, had asked their supporters to fill the streets of Caracas and other cities in rival demonstrations.

“We want to march! Yes we can!” shouted opposition protesters as riot police prevented them from accessing the street in east Caracas where their demonstration was due to take place.

Overnight, security services had stopped the opposition from setting up a stage in an avenue where their protest was due to take place.

“They think they can scare us but they will get a surprise form the people in the street,” Guaido tweeted.

“They think they can wear us down, but there's no way they can contain a population that has decided to end the usurpation,” added the leader of the opposition-controlled legislature, who is recognised as Venezuela's interim president by more than 50 countries.

Guaido is trying to force out Maduro — whose May re-election he deems illegitimate — in order to set up new elections.

Opposition lawmakers denounced the overnight arrest of three people who were setting up a stage at the opposition rally site.

Maduro has asked his backers to march against “imperialism.” “We're continuing the battle and victory over the permanent and brutal aggression against our people,” Maduro wrote on Twitter.

“Today, more than ever, we're anti-imperialists. We will never surrender!”

'Forceful response'

The mounting political pressure comes as services slowly returned to normal in Caracas and the states of Miranda and Vargas, home to the country's international airport and main port.

“The US empire once again underestimates the conscience and determination of Venezuela's people,” tweeted Maduro, who has not been seen in public since the blackout began late Thursday afternoon.

“I assure them that every attempt at imperial aggression will be met with a forceful response from the patriots who love and valiantly defend our homeland.”

The western regions of Barinas, Tachira and Zulia remained without electricity while in other states the supply was proving unstable.

It was one of the worst and longest blackouts in recent memory in Venezuela and paralyzed most of the country. Its cause is still unknown.

Hospitals had reported terrible problems and those with generators were using them only in emergencies. Flights were cancelled, leaving hundreds of travellers stranded at airports.

The Caracas subway, which transports two million people a day, remained suspended early Saturday and shops were closed, but internet and telecommunications services were returning to normal.

“The problem is food, I'd bought meat and it's going bad. I'm going to the march because we need change. We're fed up,” Luis Alvarez, a 51-year-old truck driver, told AFP.

Maduro had blamed the blackout on US sabotage and shut down offices and schools on Friday.

Large lines formed at the few gas stations open as people fetched fuel for generators. Some took gas from their cars.

Scenes of chaos

Venezuela has suffered more than four years of recession that has seen poverty soar as citizens struggle with food and medicine shortages.

Problems have been exacerbated by hyperinflation the International Monetary Fund says will reach 10 million per cent this year.

During the blackout, witnesses described scenes of chaos at several hospitals as people tried to move sick relatives in the dark to clinics with better emergency power facilities.

Marielsi Aray, a patient at the University Hospital in Caracas, died after her respirator stopped working.

“The doctors tried to help her by pumping manually. They did everything they could, but with no electricity, what were they to do?” asked Jose Lugo, her distraught uncle.

The putrid odor of rotting flesh hung around the entrance to Caracas's main Bello Monte morgue, where refrigerators had stopped working and worried relatives gathered outside, waiting to be allowed to bury their dead.

Following Maduro's decision to close the borders to keep out desperately needed humanitarian aid, the country was completely isolated on Friday.

Critics blame the government for failing to invest in maintaining the electrical grid, although the government often points the finger at external factors when the lights go out.

The state power company Corpoelec said there had been sabotage at the Guri hydroelectric plant in Bolivar state, one of the largest in Latin America. It gave no details.

US Secretary of State Mike Pompeo said Maduro was wrong to blame the US or any other country for Venezuela's woes.

“Power shortages and starvation are the result of the Maduro regime's incompetence,” he tweeted.