Former prime minister Tony Abbott has told an anti-abortion rally in Sydney that the proposed bill to decriminalise abortion is "effectively infanticide on demand".

Key points: The rally was held to oppose abortion decriminalisation laws which passed the lower house last month

The rally was held to oppose abortion decriminalisation laws which passed the lower house last month Mr Abbott and Mr Joyce spoke out against the legislation at the rally

Mr Abbott and Mr Joyce spoke out against the legislation at the rally The bill is is due back before the NSW Upper House this week

Thousands joined the rally to oppose the bill which is due back before the NSW Upper House this week for amendments to be debated.

Among them was former federal Nationals leader Barnaby Joyce, who has repeatedly weighed into the debate over the contentious bill.

Mr Abbott was scathing in response to the Berejiklian Government allowing the private member's bill to come before the parliament, saying it was not what it was elected to do.

"The first serious legislation that the new Government puts into this parliament, is for the most radical abortion laws in this country," he said.

He claimed the bill was based on "a lie" because "whether we like it or not abortion has been decriminalised in this state for up to sixty years."

He described it "as a licence for sex-selection abortions, and late-term abortions" and said it was "effectively infanticide on demand".

Protestors rally against the abortion legislation in Sydney. ( ABC News: Ashleigh Raper )

"You don't have to be a Catholic, you don't have to be a Christian, you don't have to be an evangelical, you don't have to believe in anything other than the basic decency of every other human being to think that we should never have infanticide on demand."

Mr Joyce labelled the bill "animalistic" and said it failed to recognise the rights of the unborn child.

"There is not one person here who was not born … there comes a time when there are two people, and two people have rights and those rights must be respected otherwise society is destroyed," he told the rally.

"If you want to kill someone, how do you do it? You dehumanise them, you give them another word, you don't call them a person … well I'll call them a fetus.

"Where does this animalistic process stop? This is the slavery debate of our time."

The anti-abortion rally came the day after a much smaller rally for supporters of the bill was also held in Hyde Park.