At least two people have been shot dead during a student demonstration in the Venezuelan capital, Caracas.
A pro-government activist and at least one student were killed in a shootout, officials said.
At least 23 people are said to have been injured in demonstrations across the country.
President Nicolas Maduro is facing rising criticism for the country’s economic woes and high levels of violence.
He later spoke on national TV, condemning the violence and appealing for calm.
“Let nobody go mad and accept provocations,” Mr Maduro said.
The students were demonstrating against the detention of fellow students earlier in the week on the day Venezuela celebrated the 200th anniversary of the Battle of Victory, also known as the Venezuelan Day of Youth, an historical episode in which students defended a city against Spanish-led troops.
Opposition leaders also took part in the march.
But as in other protests called by the opposition, many also took to the streets to support the government.
The president of the National Assembly, Diosdado Cabello, accused right-wing activists of carrying out the acts of violence.
“Fascism has murdered a comrade,” Mr Cabello said on national TV.
But a leading opposition congresswoman, Maria Corina Machado, said violence had erupted as students were corralled by pro-government activists.
The leader of the opposition, Henrique Capriles, also appealed for calm.
“Violence will never be the way! We are confident that a large majority refuses and condemns it,” Mr Capriles wrote on Twitter.
Students have been protesting elsewhere for a week, with the biggest crowds in the cities of Merida and Tachira.
They were complaining about a lack of security in their university campuses, as well as protesting about the country’s economic crisis.
Government forces detained a number of students, which gave rise to more protests demanding their freedom.
Venezuela has recently announced measures to address its foreign currency crisis and boost the economy.
It has also imposed a 30% cap on profit to businesses, as part of what President Maduro calls his “economic offensive”.
In November, Mr Maduro was granted special powers to govern by decree for a year to deal with the economic crisis.
Despite its oil wealth, Venezuela has faced a shortage of staple goods such as sugar, cooking oil and toilet paper.
Among the measures decreed by Mr Maduro are official controls on the price of new and second-hand cars.