Until last year India permitted the copying of patented drugs, which allowed the country's pharmaceutical industry to sell cheap versions of Aids drug cocktails, known as antiretrovirals. Legislation enacted in March 2005 curtails the ability of firms to make copycat treatments and allows foreign pharmaceutical companies to claim ownership of drugs.

The California-based Gilead and Britain's GlaxoSmithKline, have now applied for patents on two HIV treatments. Campaigners, lawyers and Indian drug makers have opposed the applications, and more than 100 people were arrested in protests yesterday in Delhi.

Activists say patents would drive up prices as Indian manufacturers would have to pay royalties and rival generic versions would be blocked for 20 years.

Gilead has sought a patent on a key Aids treatment called tenofovir (Viread), while Glaxo has sought one for a widely used drug called Combivir. Lawyers say 8,000 patent applications are in the pipeline.

Exports by Indian companies helped to cut the price of antiretroviral treatment from $15,000 (£8,000) per patient per year a decade ago to $200. Indian companies now provide two-thirds of the world's cheap Aids therapies.

Campaigners say that as Aids patients develop a resistance to "first-line" drugs, there will be no scope for a reduction in prices of second-generation medicines without the Indian generic drugs. The second-generation drugs are already 10 times more expensive than older treatments.

"Granting [these patents] would set a dangerous precedent," said Ellen 't Hoen, director of policy at Medecins sans Frontieres. "We will be back to the days when multinationals controlled the price."

Drug companies say they sell to poor countries at cheap rates and that problems with public health systems, rather than patents, curtails accessibility.