Bengal Chief Minister Mamata Banerjee Sunday scripted an astounding victory in the crucial bypoll in the eastern state to cement her position as the biggest opposition leader in India.

The 66-year-old won the by-election for the assembly constituency of Bhawanipur in state capital Kolkata by a whopping margin of 59,000 votes over her main rival, a little-known lawyer from Prime Minister Narendra Modi's ruling Bharatiya Janata Party (BJP).

Read: Mamata all set to win crucial Bengal bypoll

"I have won the Bhawanipur assembly bypoll with a margin of 58,832 votes and have registered the victory in every ward of the constituency," Mamata said, addressing the media in Kolkata.

The firebrand politician had to contest in the bypoll as she was defeated in Nandigram by her protege-turned-rival Suvendu Adhikari in April-May's assembly elections though her ruling Trinamool Congress party swept back to power in Bengal.

India's main opposition Congress, on the other hand, did not fielded any candidate against Mamata. The party's decision was announced by West Bengal Congress chief Adhir Ranjan Chowdhury earlier last month.

On May 2, Mamata single-handedly pulled off a landslide victory in the assembly election for the third time in a row, bucking anti-incumbency and staving off a massive challenge from Prime Minister Narendra Modi's BJP.

Read: Crucial bypoll begins in Bengal to decide Mamata's fate

In June this year, Mamata moved the high court to declare Suvendu's election win "null and void' on the grounds that he had indulged in corrupt practices and sought votes on the basis of religion. 

Bengal had witnessed the most high-profile contest in India's recently held state polls. While Mamata harped on being Bengal’s daughter, the BJP asked people to vote for "change and development" after 50 years of Communist and Trinamool rule.