In Buenos Aires, protesters gathered at the Plaza de Mayo, opposite the presidential palace
Thousands of Argentines have marched in the capital Buenos Aires against the disappearance of a human rights activist in the Patagonia region.
Santiago Maldonado, 28, went missing at an indigenous-rights demonstration in Patagonia a month ago.
Human rights campaigners, union leaders and left-wing groups are calling on the government of Mauricio Macri to do more to find him.
The rally was one of many protests in cities across Argentina.
Witnesses say Mr Maldonado was last seen after being arrested at a protest for the rights of the Mapuche indigenous group in southern Argentina on 1 August.
A community of Mapuche have been occupying land in the province of Chubut, claiming ancestral rights in a privately-owned area.
On the day that Mr Maldonado disappeared, border Police arrived to dismantle a roadblock that had been erected by protesters on Route 40, the main road crossing Argentina from north to south. Police deny detaining him.
Mr Maldonado's brother, Sergio, spoke at the demonstration in Buenos Aires.
"One month after Santiago's forced disappearance, the state continues to deny it. The only people they have questioned are his friends and family," he said.
He called for an independent investigation on all the police officers working on that day in the area.
The picture of the missing activist is now on shirts and posters across the country, next to the question: Where is Santiago Maldonado?
The shady past of Argentina's security forces has led many to believe that he had been kidnapped or killed.
But Argentine Security Minister Patricia Bullrich said things had changed since the restoration of democracy in the early 80s.
"The police are not the same as 40 years ago," she said.
President Macri's government has offered a reward of almost $30,000 (£23,000) for information on his disappearance.
Former president and now head of the opposition Cristina Fernández de Kirchner has repeatedly spoken out on the disappearance, which coincided with the first round of congressional elections.