(Adds lawmaker, election board, Supreme Court)

By Andrew Cawthorne

CARACAS, June 7 (Reuters) - A 17-year-old Venezuelan protester died in ferocious clashes between security forces and protesters in Caracas on Wednesday, taking the death toll from unrest since April to at least 66.

The government said Neomar Lander died when a homemade mortar exploded in his hands while hundreds of youths faced off with National Guard troops in the Venezuelan capital.

Opposition lawmakers, however, said he was killed by a tear gas cannister fired straight at him. The state prosecutor's office announced a probe, without giving details.

A Reuters photographer saw a young man, assumed to be Lander, lying bloodied and motionless on the street, receiving emergency first aid.

At least 66 people have died, with victims including government and opposition supporters, bystanders and members of the security forces, since demonstrations began against President Nicolas Maduro more than two months ago.

Each side blames the other for the violence.

Critics denounce Maduro as a repressive dictator, and are demanding general elections, foreign humanitarian aid, freedom for hundreds of jailed activists, and autonomy for the opposition-controlled National Assembly.

"We mustn't let fear intimidate us, let's stay in the streets to fight for all Venezuelans' future," said opposition lawmaker Miguel Pizarro, weeping at a news conference as he described seeing Lander's death.

"Soon we will able to say these were the last, dark days of the dicatorship."

Maduro, 54, calls his foes violent right-wing "fascist" conspirators seeking a coup similar to the short-lived 2002 toppling of his predecessor Hugo Chavez.

"This is criminal terrorism, and we must reject it," Maduro told supporters in a speech carried on state TV, comparing his foes to Adolf Hitler, Benito Mussolini and Augusto Pinochet.

ECONOMIC CRISIS

In the worst turmoil of Maduro's turbulent four-year rule, thousands of people have been injured and arrested in recent weeks. Protesters are also complaining about hunger and shortages during a brutal economic crisis in the OPEC nation.

In Caracas on Wednesday, security forces using armored vehicles, water cannons and tear gas blocked opposition supporters from marching to the national election board's headquarters, sparking clashes around the city.

Masked youths with homemade shields hurled stones and Molotov cocktails, and set up burning roadblocks, while businesses and schools closed, and residents ran for cover.

Among hundreds of injuries reported in Caracas and other cities, the government highlighted the case of two National Guard soldiers wounded by gunshots when they were clearing a barricade in the El Paraiso district of the capital.