The Prime Minister dedicated the 182-meter statue to the nation at Kevadiya in Narmada district of Gujarat on the 143rd birth anniversary of Patel.
"Ironic that a statue of Sardar Patel is being inaugurated, but every institution he helped build is being smashed", Gandhi said on social network Twitter, using an honorific for Patel.
Patel was known as the "Iron Man of India" for integrating various states in the post-independence era as the creation of Pakistan led to a massive bloodshed between Hindus and Muslims moving between the two nations.
The launch of the giant statue, which stands almost twice the height of New York's 305-ft tall Statue of Liberty, is widely seen as the unofficial launch of Mr Modi's re-election campaign. The chiefs of 22 villages near the statue signed an open letter calling on Modi to stay away from the inauguration.
Air Force planes showered flowers on the statue, which was designed by decorated Indian sculptor Ram V Sutar. The Statue of Unity, set up under the Sardar Vallabhbhai Patel Rashtriya Ekta Trust, has been constructed at a cost of ₹2,989 crore (approx $407 million) in less than four years.
Posters of Modi with Gujarat's chief minister, Vijay Rupani, were torn down or had their faces blackened at the weekend.
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"Erected on the banks of river Narmada in Gujarat, the structure is twice the height of the Statue of Liberty". He will also inaugurate the "Wall of Unity" (a monument defining India's unity) near the statue.
"The farmers of India had unparalleled faith in him... traders and industrialists preferred to work with Sardar Patel because they felt here was a stalwart who had a vision for India's economic and industrial growth", said Mr. Modi.
Politically speaking, the Modi government's move to come up with a grand statue of Sadar Vallabhbhai Patel seems like a masterstroke.
This is not the first time the villagers and local activists have been detained ahead of the prime minister's programme at Kevadia Colony.
Some political commentators have criticized Modi for his expenditure on the statue complex at a time when the adivasi indigenous people who farm the land acquired for it are in economic straits and have protested against it. It will feature a viewing gallery at the height of 153 metres and can host over 200 tourists at once.
Indian authorities hope the statue will attract 15,000 visitors a day to the remote corner of Gujarat, which is about 100 kilometres (60 miles) from the nearest city of Vadodora.