Some 1,000 people have demonstrated in Warsaw against a Polish bill that proposes jail time for anyone who "promotes or approves underage sexual activity".

The Stop Paedophilia committee behind the bill claims it will protect minors against "sexual depravation and demoralisation," but critics say it aims to ban sex education.

"I have children who don't yet go to school. I would like to be able to talk to them one day about sex. This poorly written bill prevents me from doing so. It violates the Constitution," sociologist and leftist lawmaker Magdalena Biejat told demonstrators.

The bill "creates an environment in which children won't know how to say 'no', nor will they know when to say 'yes.' One where they won't be able to talk to teachers or doctors... We will form potential paedophile victims," she added.

The bill foresees a sentence of up to three years in prison, while lawmakers from the governing conservative Law and Justice (PiS) party have proposed to increase it to five years.

The PiS, which is close to the country's powerful Catholic Church, notably campaigned on its opposition to the "sexualisation of children" before winning Sunday's general election.

Holding signs with slogans like "sexual democracy," "shame," and "PiS off," protesters gathered in front of Warsaw's parliament for the demonstration organised by a feminist group.

"I'm here to say that good sex is a good thing, and that education is needed for it to be good," Warsaw protester Ludmila said.

"I regret our Catholic culture which consists of instilling fear in people by presenting sex as a danger," the feminist and academic added.