Conversion is a highly charged political issue. Several states have passed laws this year making it harder to convert, and the mass ceremonies will infuriate Hindu nationalist parties that have been campaigning to stop lower caste Hindus changing their religion.

But for many Dalits, as Untouchables are now known, conversion is the only way to escape the oppression they still face in Hindu society. Untouchability has been illegal in India since independence, but it is still commonly practised. In many villages Dalits are not allowed to drink clean water from a well. In some areas, tea shops keep a different glass for Dalits to use, so higher-caste Hindus are not "polluted" by drinking from the same vessel, even after it has been washed. After the 2004 tsunami, Dalit survivors in Tamil Nadu were prevented from sharing water in relief camps.

Dalits are converting in large numbers this year because it is the 50th anniversary of the conversion of their most important leader of modern times, B R Ambedkar, who first called on Dalits to become Buddhists in order to escape discrimination.

When Mahatma Gandhi was leading non-violent protests against British rule, Ambedkar was using the same methods to demand equal rights for Untouchables. He was critical of Gandhi, and outspoken in his attacks on Hinduism.

"These people are converting as a protest," says Sakya Ponnu Durai, one of the organisers of the mass conversion ceremonies. But Mr Durai, a Dalit who himself converted two years ago, says he has wholeheartedly become a practising Buddhist. "After converting, I have much more satisfaction," he says.

Many of those converting are doing so to escape the menial jobs traditionally assigned to Dalits. Under the rigid rules of the caste system, it is difficult to change to a job reserved for a higher caste. Although this is no longer the case in the cities, in villages it is still practised. Many Dalits are forced to work as scavengers and latrine cleaners.

Mr Durai was more fortunate: his father was in the Indian military and was able to give him a good education in Chennai. But he says he still faced discrimination.

Even at university, Mr Durai says he was badly beaten by higher-caste students enraged that a Dalit had got better marks than them. Today, he is a federal government worker in Delhi. He is fully aware that conversions are a potentially explosive issue. Hindu nationalist parties are unhappy with the large numbers of lower-caste Hindus converting, not only to Buddhism but also Christianity.

This year several states, including Rajasthan and Madhya Pradesh, have introduced laws that anyone wishing to convert will have to obtain official permission first. Gujarat, home to some of the most hardline Hindu groups, has introduced a more controversial law under which Buddhism is considered part of Hinduism.

In a separate rally this weekend, not connected to the conversion ceremonies, thousands of Dalits plan to burn the new laws.

By a strange irony, as well as the 50th anniversary of Ambedkar's death, the conversions are taking place amid controversy over the funeral of the Dalits' most powerful political leader, Kanshi Ram. Ram had also converted to Buddhism, but some of his relatives objected when his cremation was carried out according to Buddhist rites.

Across India this month, thousands of Hindus from the former Untouchable castes are converting to Buddhism in protest at the continuing discrimination they face. Mass conversion ceremonies are being held throughout the month, from Delhi in the north, to Hyderabad in the south. Organisers are claiming that more than 100,000 people have already converted.

Conversion is a highly charged political issue. Several states have passed laws this year making it harder to convert, and the mass ceremonies will infuriate Hindu nationalist parties that have been campaigning to stop lower caste Hindus changing their religion.

But for many Dalits, as Untouchables are now known, conversion is the only way to escape the oppression they still face in Hindu society. Untouchability has been illegal in India since independence, but it is still commonly practised. In many villages Dalits are not allowed to drink clean water from a well. In some areas, tea shops keep a different glass for Dalits to use, so higher-caste Hindus are not "polluted" by drinking from the same vessel, even after it has been washed. After the 2004 tsunami, Dalit survivors in Tamil Nadu were prevented from sharing water in relief camps.

Dalits are converting in large numbers this year because it is the 50th anniversary of the conversion of their most important leader of modern times, B R Ambedkar, who first called on Dalits to become Buddhists in order to escape discrimination.

When Mahatma Gandhi was leading non-violent protests against British rule, Ambedkar was using the same methods to demand equal rights for Untouchables. He was critical of Gandhi, and outspoken in his attacks on Hinduism.

"These people are converting as a protest," says Sakya Ponnu Durai, one of the organisers of the mass conversion ceremonies. But Mr Durai, a Dalit who himself converted two years ago, says he has wholeheartedly become a practising Buddhist. "After converting, I have much more satisfaction," he says.

Many of those converting are doing so to escape the menial jobs traditionally assigned to Dalits. Under the rigid rules of the caste system, it is difficult to change to a job reserved for a higher caste. Although this is no longer the case in the cities, in villages it is still practised. Many Dalits are forced to work as scavengers and latrine cleaners.

Mr Durai was more fortunate: his father was in the Indian military and was able to give him a good education in Chennai. But he says he still faced discrimination.

Even at university, Mr Durai says he was badly beaten by higher-caste students enraged that a Dalit had got better marks than them. Today, he is a federal government worker in Delhi. He is fully aware that conversions are a potentially explosive issue. Hindu nationalist parties are unhappy with the large numbers of lower-caste Hindus converting, not only to Buddhism but also Christianity.

This year several states, including Rajasthan and Madhya Pradesh, have introduced laws that anyone wishing to convert will have to obtain official permission first. Gujarat, home to some of the most hardline Hindu groups, has introduced a more controversial law under which Buddhism is considered part of Hinduism.

In a separate rally this weekend, not connected to the conversion ceremonies, thousands of Dalits plan to burn the new laws.

By a strange irony, as well as the 50th anniversary of Ambedkar's death, the conversions are taking place amid controversy over the funeral of the Dalits' most powerful political leader, Kanshi Ram. Ram had also converted to Buddhism, but some of his relatives objected when his cremation was carried out according to Buddhist rites.