But "showing the magnanimity of the law and (in light of the fact) the respondent is a woman", Justices G.B. Pattanaik and R.P. Sethi said they would only give her a "symbolic" one-day jail sentence and a 2,000-rupee fine.

Roy, a passionate campaigner against construction of the billion-dollar dam in western India, said she had come prepared for the decision.

"I have my backpack," she said as she was escorted out of the court on her way to New Delhi's Tihar Jail, the largest prison in Asia, home to an assortment of criminals from petty crooks to extortionists and murderers.

Roy, who shot to overnight fame in 1997 when her first novel, "The God of Small Things", won the Booker Prize, told reporters she had not decided whether she would pay the fine.

If she refuses to pay, the court said she would face an additional three months in jail.

Her lawyer, Prashant Bhushan, told reporters that Roy would file a petition challenging the Supreme Court ruling on grounds that it violated principles of natural justice.

The ruling stemmed from an affidavit filed by Roy against a contempt-of-court case lodged against her last year following a public protest over construction of the dam on the Narmada River.

The Supreme Court dismissed that case but it said yesterday that the affidavit filed by Roy in response to those proceedings amounted to contempt of court.

Critics say that the dam, India's biggest hydroelectric project, will mean large-scale flooding, cause huge environmental damage and result in the displacement of millions of people.

But supporters say the dam is needed to provide electricity to large swathes of energy-hungry India.

Outside the court, demonstrators held aloft banners reading, "Criticism is not contempt".