India's opposition Congress party named Rahul Gandhi as its new leader on Monday, capping years of speculation about the scion of the dynasty that has ruled the country for much of its independent history.

The 47-year-old has long been seen as the likely successor of his mother Sonia, who took over as party president nearly two decades ago following the assassination of her husband Rajiv.

But doubts emerged after Rahul led the party to its worst ever general election defeat in 2014, gaining a reputation as a reluctant politician who lacks the ruthlessness to succeed in the cutthroat politics of the world's largest democracy.

Party workers cheered as senior Congress leader Mullappally Ramachandran announced that Rahul, who stood for election unopposed, would be the next president of the party his family has led for generations.

"I hereby declare Shri Rahul Gandhi elected as the President of the Indian National Congress," he told reporters, calling it "a historic occasion".

"I have waited for this day for so many years. My joy knows no bounds," said Prem Chowdhury, a long-time party member and former municipal councillor.

Congress party workers let off firecrackers and shouted "long live Rahul" as they celebrated his ascension.

Prime Minister Narendra Modi, who has repeatedly derided Rahul as a "shehzada" or prince, congratulated his opponent on his ascension.

"My best wishes for a fruitful tenure," Modi posted on his Twitter account.

- First test -

Rahul -- whose father Rajiv, grandmother Indira and great-grandfather Jawaharlal Nehru all served as prime minister -- has been vice-president of the party since 2013 and led its campaign for the last general election.

He was strongly criticised for Congress' worst-ever showing as it lost power to Modi's Bharatiya Janata Party (BJP).

Since the national election defeat, the party has lost polls in many states to the BJP, exposing Rahul to further criticism.

But few inside the party have been willing to publicly criticise the family that has been at its helm for generations.

Rahul faces his first test as Congress leader in elections in Gujarat, Modi's home state, which the BJP has ruled for over two decades.

He has led the Congress campaign for the election, winning the backing of some influential caste groups who say they have been left behind by an economic boom in the prosperous western state, although Modi remains the man to beat.

"Everyone is waiting to see what he can do for the fortunes of the Congress," Delhi-based political anaylst Manisha Priyam told AFP.

- Reluctant leader -

Sonia Gandhi wielded huge influence over Congress administrations despite having no official government role.

She was thrust into the cauldron of Indian politics in 1998 when she agreed to take over the leadership of the party, seven years after the assassination of her husband Rajiv Gandhi.

She overcame stagefright to propel Congress to a surprise electoral win over the BJP in May 2004, addressing huge rallies to cheers of "Sonia Gandhi zindabad" -- "Long Live Sonia Gandhi".

The Italian-born leader spoke in Hindi, reading from Romanised text, telling audiences that her heart was "buried in the soil of this country."

"She understood India very well. She was successful in blunting allegations of being an outsider and history will judge her very well on that count," Gandhi family biographer Rasheed Kidwai told AFP.

But her health has been the subject of much speculation in India since 2011, when she went to the United States for surgery.

Kidwai said Sonia may keep her parliament seat but pull back from day-to-day politics, and warned that her reputation would rest on her son's performance.

"If Rahul fails and falters, or Congress gets fragmented, then this would give ammunition to critics (who say) that Sonia imposed her son on the party," he told AFP.

Some Congress supporters have called for Rahul's sister Priyanka to take a more prominent role, but she has always refused.

Party supporters, however, say Rahul has displayed greater political acumen in recent years.

He will officially take over as Congress president on Saturday at a ceremony in New Delhi.

Source: AFP