Mrs. Gandhi's feisty daughter-in-law: more than a political nuisance?

It was an extraordinary performance. The diminutive widow, dressed in a mourning sari of white, stood before thousands of villagers in the poor farming community of Amethi, in northern India.

''Three years ago, I came to you as a bride,'' said Maneka Gandhi, at a rally last fall. ''Today, I come as a widow who, with a small child, was thrown out of her mother-in-law's house.''

It was theater of the highest order, and the tale quickly spread throughout the sun-baked villages of Uttar Pradesh.

This was no ordinary widow. It was Prime Minister Indira Gandhi's daughter-in-law: ambitious, cunning, convincing - a political creature from head to toe.

On Sunday, the remarkable family saga came full circle when 26-year-old Maneka launched an opposition political party, named after her late husband, Sanjay. And, in the dynastic intrigue that so titillates the Indian masses, her challenge was direct. She would stand against Mrs. Gandhi's other son and heir apparent, the stolid Rajiv, for Sanjay's coveted Amethi parliamentary seat.

Maneka denies any ambition to become a new imperious queen for any Gandhi dynasty. ''A dynasty is rubbish,'' she said in an interview. ''Indians will no longer accept it. With 61 percent of the people living below the poverty line, they want bread, someone to fill their stomachs. They no longer need spectacles, pomp and circumstance.''

Dwarfed by a large, crescent-shaped desk in the office of a trucking firm that was started by her husband but that she now operates, Maneka Gandhi smiled impishly when asked about her mother-in-law. ''She has me under 24-hour surveillance. My telephone is tapped. My mail is censored. And my supporters are constantly in and out of jail.''

Since the death of the controversial Sanjay, his mother's favored son, in an aerobatic plane crash in January 1980, the two strong-willed Gandhi women have sanctioned a war of words, evoking scenes from ''The Women'' of Clare Booth Luce.

During her whirlwind marriage, the impetuous Maneka thrived on the political limelight. Then she went into two years of mourning, wearing only gray sackcloth. She spent hours watching video movies; her favorites were ''Casa-blanca'' and ''Amarcord.''

With annoyance and dismay she watched the cautious advance of Rajiv into the political arena. Her husband's ambitious followers had begun paying court. Sanjay's legacy must be perpetuated, they insisted, and it must be perpetuated through her.

The growing rift within the sprawling Gandhi complex at 1 Safdarjang Road burst into the headlines in March 1982 when the usually unflappable prime minister ordered Maneka out of her house for aligning herself with Rajiv's opponents. In a blaze of publicity, Maneka stage-managed the departure. She refused to return to her mother's house, amazing all of India's matrons by checking into a motel.

Today the freckle-faced former model, former journalist, and former writer of anthologies looks more like a teen-ager than a widow with a 3-year-old son. She dismisses as childish the scurrilous attacks against her waged by supporters of Mrs. Gandhi's Congress-I Party.

''Whenever you throw mud, you lose ground,'' she said. But it was clear that Maneka Gandhi - who has abandoned blue jeans for traditional Indian dress - relishes all of the publicity. She appears certain that her day is near.

Western diplomatic officials and Indian political pundits do not dismiss her out of hand, despite charges of her opponents that she is too young to be taken seriously and that she is politically immature. She has rallied seven members of state assemblies and two members of Parliament to her side. She claims a party membership of 800,000 and is beginning to develop a political think tank.

Some experts say she can defeat Rajiv Gandhi for the Amethi parliamentary seat, which would be far more than a nuisance to the grand dame of Indian politics. It would be a humiliation for the Gandhi dynasty.

When asked if she herself was not a creation of the prime minister, a pupil who had learned her political lessons at the official residence, Maneka laughed and acknowledged: ''Yes, that is a problem. So many people say that I'm exactly like her. . . . I did learn a great deal from Mrs. Gandhi. I learned what not to do. I learned not to distrust people to the point of paranoia; not to surround myself with second-rate advisers who tell me what I want to hear; and, most importantly, to learn economics. Forty-six million people in this country are unemployed. Well over half are living below the poverty line.''

There is one thing that novice politician Maneka Gandhi does not have to fear , and that is lack of name recognition. Her sparring match with the prime minister has made her one of the best-known institutions in India today.

Said a sociologist specializing in Hindu family life, ''Every daughter-in-law in this country is, in one way or another, abused. But you just don't expel her from the family home. Thus, Indira Gandhi, literally overnight, ceased being the 'ammah' (mother) of India, and became the mother-in-law.''