WELLINGTON, Nov 19 (Reuters) - Tens of thousands of New Zealanders rallied in front of parliament on Tuesday in one of the country's largest ever protests to oppose a bill, which opponents say seeks to dilute the rights of Maori and threatens to set race relations back decades.

Massive crowds estimated by police at 42,000 gathered at parliament where the Treaty Principles Bill was introduced earlier this month by legislators who want to reinterpret a 184-year-old treaty signed between the British and Indigenous Maori.

The libertarian ACT New Zealand party, a junior partner in the ruling centre-right coalition government, is seeking to enshrine a narrower interpretation of the Treaty of Waitangi that it says discriminates against non-Indigenous citizens.

While the bill lacks enough support to pass, critics see in it a desire to reverse decades of policies that aimed to empower Maori, who make up around 20% of the 5.3 million population but have higher levels of deprivation and incarceration and worse health outcomes than the broader population.

"I'm here for my grandchildren, my children and for their children", said Hoana Hadfield from Wellington, who was marching in a protest for the first time.

"I think it's important that we keep our kaupapa which is our values as Maori and our culture and its a real big thing for us to have cultural identity."

Some people in the crowd were dressed in traditional attire with feathered headgear and cloaks and carried traditional Maori weapons, while others wore t-shirts emblazoned with Toitu te Tiriti (Honour the Treaty). Hundreds carried the Maori national flag.

The protest was preceded by a nine-day march, or hikoi, that began in the country's far north, with thousands joining rallies in towns and cities as marchers travelled south on foot and in cars to Wellington.

Unity and solidarity was spoken about a lot in the rally. Helmut Modlik, a leader in the Ngati Toa tribe, told the crowd that it was too late to divide the country.

"To those who will divide us, whanau, it is too late. We are already one people," Modlik said to cheers from those gathered on the lawns of parliament.

First signed in 1840 between the British Crown and more than 500 Indigenous Maori chiefs, the Treaty lays down how the two parties agreed to govern.

The interpretation of clauses in the document still guides legislation and policy today, with rulings by the courts and a separate Maori tribunal expanding Maori rights and privileges over the decades.

ACT's coalition partners, the National Party and New Zealand First, agreed to support the legislation through the first of three readings, which happened last week. However, both have said they will not support it to become legislation.

A small number of politicians including ACT leader David Seymour came onto the forecourt of parliament to listen to the protesters speak. A petition opposing the bill of 203,653 signatures was also presented to parliament.

When Seymour emerged thousands started chanting "kill the bill" which was followed by a haka, or Maori war dance.
Abby Collier, 42, had travelled six hours from her home in Tairawhiti to participate in Tuesday's protest. She said she was supporting the movement because she believed unity was important.

"We are coming from across the country and showing our babies we can have a positive impact through kindness," Collier said.