Thousands of people have demonstrated in the Argentine capital Buenos Aires in protest at the murder of a left-wing activist during a labour dispute.
Crowds blocked roads and railway lines and gathered outside the presidential palace to demand justice.
The activist, Mariano Ferreyra, was shot dead during a clash between rival groups of railway workers.
One of the country's main trade union federations has called a general strike.
Left-wing opposition groups have blamed the killing on members of the main railway workers union, which they say was acting to break up a protest against low pay and job losses in contractors that provide services to the state railway company.
Eyewitnesses said Mr Ferreyra, 23, was shot in the head at close range when a group of protesters from the left-wing Workers' Party was attacked.
The Workers' Party alleges police stood by while the violence took place.
President Cristina Fernandez de Kirchner has expressed regret at his death, saying she "does not want to live in a society where people go out to protest with sticks and weapons".
But she said her government would maintain its policy of "not repressing" social protests.
The leader of the CTA trade union federation, Hugo Yasky, said it was time to end "the complicity between business groups and trade unions which act as their shock troops".
The railway workers union, members of which are alleged to have carried out the attack, belongs to the rival CGT federation.