Tens of thousands of people demonstrated in Buenos Aires on Tuesday against the new Argentine government's reforms, including measures they say curb press freedom.

Crowds responding to an online call by organizers swamped the Plaza de Mayo square in front of the presidential palace in the Argentine capital.

The demo was called as a protest against President Mauricio Macri's bid to repeal a law by his leftist predecessor that outlawed monopolies by media companies.

It expanded to draw crowds of protesters angry at the treatment of a prominent journalist and critic of Macri who was fired on Monday from a private radio station.

Macri called the fired journalist, Uruguayan-born Victor Hugo Morales, a "fanatic Kirchnerist."

But he insisted: "This government is not looking to see what journalists work in what media."

The crowd waved signs reading "No to censorship. Freedom!" and yelled: "Macri, you trash, you are a dictator."

A 2009 media law introduced by Macri's predecessor Cristina Kirchner aimed to break up what she described as media monopolies, such as the powerful Clarin group that fiercely criticized her.

Macri is fighting in the courts to push through a presidential decree to overturn that law. His side is outnumbered by his political opponents in the state legislature.

It was the latest, and one of the biggest, in a series of street protests against Macri since he took office a month ago.

On Friday, police fired rubber bullets and tear gas at protesting municipal workers in La Plata, south of Buenos Aires, in a dispute over public sector layoffs.

Macri has pushed through a series of economically liberal reforms to reverse Kirchner's policies.

These included freeing up foreign trade and currency controls, moves that critics say will hurt poorer Argentines.