(Recasts with latest from marches, trouble on street)
By Daniel Wallis and Efrain Otero
CARACAS, Feb 12 (Reuters) - One person was killed during standoffs at the end of an anti-government rally in Caracas on Wednesday, witnesses said, escalating the worst bout of unrest in Venezuela since protests against President Nicolas Maduro's April 2013 election.
A Reuters cameraman and a photographer heard shots and saw one protester had fallen to the ground. The man was then carried away dead, the witnesses said, apparently shot in the head.
With both pro- and anti-government demonstrations underway in the city, neither the identity of the man nor the gunman were immediately clear.
But National Assembly head Diosdado Cabello said he was a government supporter. "He's a comrade assassinated by the right-wing fascist hordes," he said in a speech.
The fatality came at the end of rival demonstrations to support and denounce Maduro's government after several weeks of opposition street protests.
The raucous and colorful competing rallies in the Venezuelan capital had passed off largely peacefully, until some 100 young opposition supporters began burning tyres in the streets, witnesses said.
Police responded with teargas and rubber bullets.
Under the banner "The Exit," meaning Maduro's departure, hardline opposition groups have been holding mostly small protests around the country for the last two weeks, to complain about rampant crime, corruption, and economic hardships.
Some have degenerated into rock-throwing skirmishes with security forces in the first sustained unrest since last year's post-election riots that killed half a dozen people.
Maduro, a 51-year-old former bus driver and union activist who has pinned his presidency on maintaining the legacy of late socialist leader Hugo Chavez, says right-wing "fascists" are seeking to destabilize his government and topple him.
"A Nazi-fascist current has emerged again in Venezuela, they want to lead our nation to violence and chaos," Maduro told pro-government demonstrators clad in the red colors of the ruling Socialist Party at their rally.
"Do not fall for their provocations!" he said in a speech just minutes before the violence.
A few blocks away, thousands of opposition supporters had gathered in a square and marched toward some government buildings, chanting and blowing horns and whistles.
"We don't want a Cuban dictatorship," some chanted, in a reference to accusations that Maduro is a stooge of Cuba's President Raul Castro and his brother Fidel.
"You need therapy to live in Venezuela!" read one banner, held up by members of a university psychology department.
ARRESTS AND INJURIES
The marches were held as part of Wednesday's "Youth Day" commemoration that celebrates the participation of students in a 19th century independence battle against colonial authorities.
"We are the young revolutionaries, hand-in-hand with the Venezuelan government," shouted one Maduro supporter at the pro-government rally in Caracas.
About 20 demonstrators have been arrested since Popular Will and another hardline opposition group began calling for street protests to force Maduro's exit from power two weeks ago.
Opposition activists said armed pro-government supporters belonging to militant community groups known as "colectivos" attacked and shot at people protesting in the western Andean city of Merida on Tuesday, injuring five.
"They were attacked by the colectivos while exercising their right to peaceful protest," said Tamara Suju, who is tracking the violence for Popular Will.
Maduro blamed the Merida incident on opposition provocateurs posing as Socialist Party sympathizers.
"They cannot take us back to the scenes of 2002," Maduro said in a reference to massive street protests that culminated in a brief, military-led coup against Chavez.
Some student protesters in the state of Merida have thrown rocks and blocked roads. Policemen have been injured during student-led protests in another western state, Tachira.
The current protests in Venezuela have been much smaller than the 2002 wave. Many in the opposition favor a more moderate approach.
"While there are plenty of reasons to protest, there does not seem to be an agenda for the current wave. #LaSalida (The Exit) is not a strategy, it's a hashtag!" complained the anti-government blog Caracas Chronicles.
"The street protests, along with the public bickering they are engendering, are creating a false sense that our actions can undo the regime, while at the same time casting doubt on the opposition's unity." (Additional reporting by Peter Murphy, Deisy Buitrago, Carlos Rawlins, Brian Ellsworth, Eyanir Chinea, Marco Bandres and Andrew Cawthorne in Caracas; Writing by Andrew Cawthorne; Editing by Brian Ellsworth and Diane Craft)