Thousands of women protested on Saturday against a bill advancing in Brazil's conservative Congress that would equate abortions after 22 weeks of pregnancy to homicide and establish sentences of six to 20 years in prison.

The demonstrators marched along Sao Paulo's main Paulista Avenue carrying banners rejecting the proposal, which they call the most repressive approach to women's reproductive rights in decades.

People of all ages, including many retirees and children, filled the streets chanting, "A child is not a mother, a rapist is not a father."

Abortion is allowed in Brazil only in cases of rape, fetal deformation or when the mother's life is in danger. If the bill backed by evangelical lawmakers becomes law, abortions by rape victims would be considered homicide after 22 weeks gestation.

Feminist groups criticized the proposed legislation for imposing harsher penalties than those given to rapists in Brazil.

They also argue that the changes would greatly impact children abused by family members. Such children, often lacking the understanding or support to recognize themselves as crime victims, frequently discover their pregnancies late.

Leftist President Luiz Inacio Lula da Silva called the bill "insane" and said his government will defend the current laws that punish rapists and treat their victims with respect.

"It is insane to want to punish a woman with a greater penalty than the criminal who committed the rape," Lula said at a news conference at the Group of Seven (G7) summit in Italy.

Protests began on Thursday in Brazil's largest cities after the lower chamber of Congress voted to put the bill on a fast track for approval, which curtails debate on the proposal.

Faced with criticism that rape victims seeking abortions could face worse punishment than rapists, the bill's author Sostenes Cavalcante said he will propose harsher sentences for rape, currently up to a maximum of 10 years in jail.

Cavalcante is an evangelical pastor and member of former hard-right President Jair Bolsonaro's party.

The angry reaction on social media may slow the bill's progress, with Speaker Arthur Lira no longer planning to put the proposal to the vote in plenary any time soon and expecting its text to be changed, a source in his office said.

Passage is even less certain in the upper chamber where right-wing senators have less clout, and Senate president Rodrigo Pacheco has said the bill must be debated in committees.

First Lady Rosangela da Silva criticized the bill and said on social media: "Congress should be working to guarantee access to legal and safe abortions through the national health system SUS."

Brazil's restrictive abortion laws mean many Brazilian women seeking to end pregnancies resort to unsafe illegal abortions and botched procedures, which cause dozens of deaths every year.