NEW DELHI - An Indian student from the lowest Dalit caste was hacked to death and his wife critically injured in southern India in a suspected "honour killing" by relatives angered by their marriage, police said Monday.

Three men armed with sickles and sharp weapons attacked the 22-year-old student and his wife, who is from a higher caste, on a crowded street in Tamil Nadu state on Sunday.

Local police commissioner N Manjunatha said the 19-year-old woman's relatives were angered by the couple's marriage. "They married some eight months ago and the woman's family was unhappy. She is an upper Thevar Hindu caste and the man was a Dalit," Manjunatha told AFP.

The woman is recovering at a local hospital and police are searching for her uncle in connection with the attack, he said. CCTV footage of the incident broadcast on Indian television showed the couple walking along the street when three men on a motorbike stop and attack them.

Thevars are a dominant community in Tamil Nadu while Dalits, formerly known as untouchables, are a historically marginalised community. India has long witnessed so-called honour killings, where couples are targeted because their families or communities disapprove of their relationships over caste or religion. Most occur in rural pockets of the north.

They are carried out by relatives or caste groups to protect what is seen as the family's reputation and pride. There are no India-specific figures available, but United Nations statistics say 1,000 out of the 5,000 such killings every year are in India. India's Supreme Court ruled in 2011 that those involved in honour killings should face the death penalty.

Reuters Adds: "This is not an ordinary murder," P. Sampath of the campaign group Tamil Nadu Untouchability Eradication Front told the Thomson Reuters Foundation.

"The bold attack happened because a young boy married a girl who was from a different community, a higher one in the social ladder. It should be investigated accordingly." Despite India's growing cosmopolitanism, discrimination against low-caste communities persists. The intermingling of caste and religion remains a taboo in Indian marriages - not only among rural populations, but even for educated, well-off families in urban India.

"A section of youth are made to feel that it is their responsibility to defend the caste honour by controlling women in their families and communities," said activist Vasuki of the All India Democratic Women's Association. "And the killing is supposed to be a 'lesson' to other girls in the community," she said, referring to women who choose to marry someone outside of their community.

Government figures show that 18 "honour killings" were reported in India in 2014. Activists say the crime is under-reported and many such killings are covered up and made to look like suicides.

"In Tamil Nadu, we have documented 81 honour killings since July 2013," said A. Kathir of the non-governmental organisation Evidence.

But according to the government of Tamil Nadu, no "honour killings" have taken place in the state in the last five years. Activists said Sunday's attack was a clear example.