Indian school children pay homage to a portrait of Indian independence icon Mahatama Gandhi, on Martyr's Day to mark the 70th anniversary of Gandhi's assassination, in Chennai, January 30, 2018. Getty

New Delhi — As India celebrated the 150th birthday of Mahatma Gandhi this week, world leaders paid tribute to the icon of the country's non-violent battle for independence, but India's lingering sectarian tension was on display. A portrait of the man who helped bridge the divide between India's Hindus and Muslims was defaced at a memorial in the center of the vast country on Wednesday. An urn containing some of Ghandi's ashes was also allegedly stolen from the site.

Unknown assailants, now the subject of a police investigation, wrote "deshdrohi" (traitor) on Gandhi's portrait at the Bapu Bhawan memorial in Madhya Pradesh state's Rewa district. The regional police confirmed to CBS News that they were investigating the defacing of Gandhi's portrait.

"We have filed a case against unknown persons; the investigations are underway but there is no progress so far," police superintendent Abid Khan told CBS News.

Gurmeet Singh Mangu, a leader of the Indian National Congress party who filed the police complaint, has claimed that an urn containing Gandhi's ashes was also stolen, but the police didn't appear convinced two days later.

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"We are not investigating the case of missing ashes or the urn," Khan told CBS News. "Gurmeet Singh has written so in the complaint. He will have to tell us where was the urn."

Singh told CBS News that when he went to the memorial to offer tributes on Gandhi's birthday, he found the urn missing.

The police have registered a case alleging "imputations, assertions prejudicial to national integration," "intentional insult with an intent to provoke the breach of peace," and "statements conducing public mischief."

A beloved icon, but not to everyone

Gandhi is celebrated by most Indians as the "father of the nation" for leading a non-violent freedom movement against British colonial rule for decades. But several right-wing Hindu nationalist groups view Gandhi as a traitor for promoting Hindu-Muslim unity, and opposing the idea of India as a Hindu nation.

His firm stand for India's secular status led to his assassination by a Hindu extremist, Nathuram Godse, in January 1948, soon after India gained independence from British colonial rule in August 1947.

Mahatma Gandhi is led down a road by his grandson, Kanubhai Gandhi, during the former's iconic Salt Satyagraha protest against British colonial taxation in Mumbai, in 1930.

There have been several attempts in the recent past by nationalists to praise the killer, Godse, including one to found a temple in his name. India's ruling party, the Hindu nationalist BJP of Prime Minister Narendra Modi, had to face the wrath of the opposition earlier this year after two of its legislators praised Godse.

"Just heard the news that someone/people stole the funeral ashes of my great grandfather kept in a shrine at Gandhi Bhavan in Rewa Madhya Pradesh. Wish he wasn't the father of the nation, wish he wasn't Mahatma. Wish he was only my great grandfather," Gandhi's great-grandson Tushar Gandhi said Friday in a statement.