Indian opposition surprises with election wins but Modi's party unruffled

India's opposition Congress party and some regional allies made a surprisingly good showing in slew of by-elections for seats in parliament and state assemblies on Thursday, raising the prospect of a challenge for Prime Minister Narendra Modi.

In four by-elections for the federal parliament and 10 for state assemblies, Modi's Bharatiya Janata Party (BJP) only retained one of each, thanks in large to united front put up by opposition parties.

After a series of humiliating defeats, the Congress party, led by Rahul Gandhi, the scion of the Nehru-Gandhi dynasty, and some regional leaders have joined forces to take on Modi.

In Uttar Pradesh, India's most populous state with 220 million people, the BJP lost both its parliament and assembly seats, in a blow to Modi, who campaigned extensively in the state.

A BJP spokesman, GVL Narasimha Rao, played down any threat to Modi, telling Reuters the opposition had tried to divide voters on caste lines.

"In the next general election, however, positive governance will play a dominant role and help Prime Minister Narendra Modi win a big mandate as the BJP is the nation's choice for positive politics and governance," Rao said.

Modi came to office in 2014 with the biggest election mandate in three decades, and has gone on to win a series of state elections since then.

Stung by Modi's electoral juggernaut, opposition parties have recently banded together in the hopes of mounting a serious challenge to the BJP in the next general election, due by May next year.