More than 7,000 people have signed an online petition saying that building tunnels under two temples, which contain "holy fire", would desecrate their sanctity, forcing nature to exact its revenge.
It claims that part of a tunnel will pass directly below the "Sanctum Sanctorum where the holy fire is enthroned" of the Wadia temple in the south of India's commercial capital.
The Change.org petition, which has received upwards of 7,290 signatures since being posted on Wednesday, says the fires are "consecrated entities; living vibrant beings who are intermediaries between God and mankind".
"To keep these holy entities safe from physical and spiritual pollution, they are segregated from the outside world by magnetic circuits which are drawn at the time of their enthronement," it explains.
"Once the tunnel for the Metro is dug under the premises, the magnetic circuits will be breached and the negative forces of physical and spiritual pollution will attack the holy fires with full force, thus diminishing their spiritual powers.
Decline in population: Of Zoroastrian descent
"If these Holy Fires are defiled, the backlash from Nature will not spare those responsible!" the petition adds.
Known as Parsis, Zoroastrians first arrived in India more than 1,000 years ago after fleeing persecution in Persia.
They became one of India's wealthiest communities, boasting a number of famed industrialists including the Tata family synonymous with the financial rise of Mumbai.
But their numbers are rapidly dwindling. Across India, where most Zoroastrians live, their numbers have halved since 1940 to around 60,000.
They believe in the god Ahura Mazda and follow the teachings of the ancient Prophet Zoroaster, viewing fire as the symbol of god's purity.
The 231-billion-rupee ($3.6 billion) line called Metro 3 will link the popular tourist destination of Colaba in the south to Seepz, a special economic zone situated 33.5 kilometres north.
The line is expected to be completed by the end of 2021. It is hoped it will ease the burden on Mumbai's notoriously overcrowded trains and help reduce traffic on its heavily congested roads.
But the project has faced considerable opposition in court, with environmental activists opposing the cutting down of hundreds of trees to make way for the line.
They also object to plans for a car park on a 30-hectare site in Aarey Colony, a biodiversity hotspot which borders the Sanjay Gandhi National Park, home to leopards and other animals.
A spokesperson for Mumbai Metro Rail Corporation Limited was not reachable for comment.
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