Tens of thousands of people demonstrated in Colombia's main cities on Sunday against the left-wing government of Gustavo Petro, whose popularity is at an all-time low after more than 18 months in power.

Demonstrators chanted against reforms he is trying to implement, including to nationalize health services, and violence that continues to mark the troubled peace talks with armed guerrilla groups.

"I voted for change, for Petro, but we're still in the same situation. I'm demonstrating because I think Colombia still has hope and because I love my country," Martha Estrada, a 64-year-old pensioner wearing a tricolor hat in Bogota, told AFP.

In the capital, tens of thousands of demonstrators braved heavy rain to make their way to Bolivar Square, next to the presidential palace.

Demonstrators also massed in cities such as Cali, Medellin and Bucaramanga.

Petro's healthcare reforms in particular have proved a lightning rod for criticism.

The government aims to reduce private sector participation in the provision of health services and pension payments, as well as to extend benefits to workers.

Experts agree that the healthcare system is bankrupt and needs to be reformed, but some question how the government intends to do so.

Petro came to power in 2022 as the first leftist to govern a country traditionally run by conservative elites.

But he lost majorities in the legislature a few months after his inauguration, and his approval rating has plummeted.