India's Supreme Court on Monday ordered a halt to the cutting of trees in financial capital Mumbai for an ambitious subway project that has sparked protests from activists opposed to the felling to build a train parking shed.

"We direct the Maharashtra Government not to cut any more trees in Aarey," Justice Mishra, heading the two-judge bench, said, requesting that Maharashtra state maintain the status quo until the next hearing on Oct 21.

Activists, Bollywood stars and some politicians have opposed cutting down some 2,700 trees in Mumbai's few green spaces, saying it would exacerbate pollution levels.

Authorities counter that there is no other viable location for the shed in the crowded city of some 20 million and proponents say the metro will be far more beneficial to the environment than the trees, warning that Line 3, slated to open in December 2021, could now be delayed due to the opposition.

It was not immediately clear how many trees had already been cut down on Friday. Police detained dozens of activists trying to protect the trees.

The Indian Express newspaper, citing senior officials, reported that 2,134 trees were already gone. The Mumbai Metro Rail Corp Ltd (MMRCL) did not immediately respond to a request for information.

The controversy over the trees has become a political issue in Maharashtra, which is holding state elections on Oct 21. Prime Minister Narendra Modi's local ally, the Hindu nationalist Shiv Sena party, has criticised the decision to cut the trees.

The Mumbai Metro 3 is a 35-kilometre project due to carry an estimated 170,000 people a day between city's business districts and the far-flung suburbs.

An average of ten people, mostly the poor, now die every day on Mumbai's overcrowded trains, according to the government.