Maori “haka” chants rang out across New Zealand’s capital as tens of thousands rallied against a conservative push to redefine the nation’s founding treaty that critics say threatens Maori rights.

More than 42,000 demonstrators poured into the capital city of Wellington on Tuesday, police said, shutting down busy streets as their spirited procession inched its way towards parliament.

Bare-chested men draped in traditional feather cloaks were joined by horse riders waving the red, white and black Maori flag. Children marched alongside adults bearing distinctive full-face Maori “moko” tattoos and clutching ceremonial wooden weapons.

Protests have been swelling throughout New Zealand after a minor party in the conservative coalition government drafted a bill to redefine the 1840 Treaty of Waitangi.

Although the bill has almost no chance of passing, its mere introduction has triggered New Zealand’s largest protests in decades.

Many critics, including some of New Zealand’s most respected lawyers, see it as an attempt to strip long-agreed rights from the country’s 900,000-strong Maori population.

Crowds gathered on the lawns outside New Zealand’s beehive-shaped parliament building, playing reggae music and giving speeches that urged the government to come out and face them.

Many demonstrators arrived in Wellington after a nine-day “hikoi” – or protest march – that began hundreds of kilometres away at New Zealand’s northern tip.

At the centre of the outcry is government minister David Seymour, the outspoken leader of the libertarian ACT Party, a minor partner in the governing coalition.

Seymour has long railed against affirmative action policies designed to help Maori, who remain far more likely to die early, live in poverty or end up in prison.

His bill would look to wind back these so-called “special rights”.

Prime Minister Christopher Luxon has voiced his opposition to Seymour’s bill, meaning it is all but doomed to fail when it comes to a parliamentary vote.

But former conservative prime minister Jenny Shipley said just putting it forward threatened to “divide New Zealand in a way that I haven’t lived through in my adult life”.

The bill was introduced to parliament for preliminary debate last week.

Proceedings were derailed when 22-year-old Maori Party MP Hana-Rawhiti Maipi-Clarke took to her feet in the chamber, ripped the bill in half and launched into a haka.

Seen as the country’s founding document, the Treaty of Waitangi was signed in 1840 to bring peace between 540 Maori chiefs and colonising British forces.

Its principles today underpin efforts to foster partnership between Indigenous and non-Indigenous New Zealanders and protect the interests of the Maori community.

The anniversary of the treaty’s signing remains a national holiday.