Former Gujarat chief ministers Suresh Mehta and Shankersinh Vaghela have also come out against the extravaganza.
In another tweet, Gandhi accused the government of "treason", saying there was a "systematic destruction" of institutions that Sardar Vallabhbhai Patel had helped in building. The current tallest statue in the world-the 502ft high Spring Buddha Temple in China-will be eclipsed by the 597 ft.
The statue has been sculpted by the famous Indian father-son duo of Ram and Anil Sutar, noted for their depictions of various Indian politicians. They were detained before Modi visited the tribal area to inaugurate the multi-crore Sardar Sarovar Project ahead of the 2017 Gujarat Assembly elections.
Recalling Patel's contribution in uniting the nation, Modi said the leader ensured "the merger of all princely states with the Dominion of India".
The Statue of Unity website, which has given the structure the tagline, "the statue as grand as the stature", says: "Built in less than five years, it is the world's tallest, grandest and giant statue".
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The 182-metre imposing monument is twice the height of the Statue of Liberty in the United States and is built on an islet, Sadhu Bet, near the Sardar Sarovar Dam in Gujarat's Narmada district.
The Gujarat government-led celebrations gripped the State under Ekta Yatra, which spread awareness about Sardar Patel, the statue and the event being held at Kevadia on Wednesday. The Communist Party of India has criticised the government for splurging money on the statue.
Prime Minister urged people to participate in "Run for Unity" in large number. It will feature a viewing gallery at the height of 153 metres and can host over 200 tourists at once.
The monument will have a museum with 40,000 documents, 2,000 photographs and a research center dedicated to Patel's life and work.
Indian authorities hope the statue will attract 15,000 visitors a day to the remote corner of Gujarat, which is about 100km from the nearest city of Vadodora.
Almost 100,000 tonnes of concrete and steel make up this giant Statue of Unity at a cost of $430 million (€378 million).