Prominent Indian rights activists have withdrawn their support for Amnesty India amid allegations of caste discrimination and harassment within the organisation, the Guardian has learned.

The allegations include claims that staff were humiliated, ill-treated and discriminated against because of their caste, a system of social hierarchy among Hindus.

A further 32 Indian lawyers and civil rights campaigners have written to Amnesty International’s secretary general in London to demand an independent investigation into the claims from staff members. Parts of the letter were seen by the Guardian.

It comes after the human rights group released a report last week identifying a “toxic” work culture at Amnesty International with widespread bullying, discrimination, public humiliation and other abuses of power. The review of workplace culture was prompted by staff concerns after researcher Gaëtan Mootoo killed himself in his Paris office.

Amnesty India (AI) said it is “deeply troubled” about the claims made by two senior campaigners and has launched an investigation, chaired by an independent expert.

One of the two campaigners resigned in protest last year after her workplace discrimination and harassment claim was dismissed by AI. The second said she was forced to leave after her project was closed, a few months after her allegation of caste-based discrimination was dismissed as unproven.

Caste-based discrimination is illegal in India, where 200 million people belong to lower caste, underprivileged communities.

Prejudice nonetheless continues, particularly in rural areas. Last year, 11 people died after violence erupted when hundreds of thousands of Dalits, formerly known as “untouchables”, took part in a mass protest to highlight discrimination they face.

Raja Vemula, son of Radhika Vemula, a leading activist on Dalit rights, said his mother has withdrawn her support for AI until the organisation addresses discrimination claims.

“Amnesty India and the international secretariat should properly examine the claims that voices of lower caste people inside the organisation have been suppressed,” he said.

Chandrasekhar Azad, a rising Dalit leader in Utter Pradesh, who was arrested and imprisoned by the Indian government last year without trial before being granted bail in November, has also withdrawn his support.

Mariya Salim, a women’s rights campaigner who worked for AI for eight months before resigning in September, said she felt “humiliated, excluded and denied opportunities unfairly”.

Salim, 31, said she knew of other, lower caste staff who had suffered discrimination but did not speak out for fear of losing their jobs. She was vilified for speaking out, she said.

“At Amnesty I was earning a good salary, but I chose dignity over money. When I resigned, I had to go back to to the Muslim ghetto, in Calcutta. We are the most marginalised people in India.

“The management at AI, mostly staffed by individuals of a higher caste and class background, is in contrast to the values of Amnesty International.”

Another former campaigner said: “It has been a horrific experience for me. I haven’t ever faced the sort of outright discrimination I found at Amnesty India. Every day, I had to prove myself – opportunities were denied to me.”

Amnesty, she said, have “lost credibility,” particularly among the marginalised people whom the organisation is supposed to represent. She is appealing against the not-proven findings in her case.

The AI report into her complaint found that a senior manager responded to a suggestion that a lower caste person should be hired to work on caste rights by saying: “An animal need not be hired to work on animal rights”.

The report concluded: “The committee is of the opinion that it is not caste-based discrimination”. Instead, it said the manager had a “behavioural problem”.

Dr Sylvia Karpagam, a doctor who left AI in 2017 after working there for eight months, said she resigned due to “organisational resistance” to addressing caste.

Aakar Patel, head of Amnesty India, said the organisation was “deeply troubled” by the allegations. A committee chaired by Dr Syeda Hameed, an independent expert, was set up in October 2018 to investigate claims that campaigners’ allegations were not properly addressed and assess practices and procedures.

Patel denied the second campaigner’s allegations that she was forced to leave after being seen as a “troublemaker.” AI had been forced to cut its workforce by 30% – 68 employees – he said.

“These decisions were completely unrelated to individual performance,” he said. “We remain deeply regretful that we had to take these decisions.”

Patel said he would “reach out” to activists who had withdrawn support.

Kumi Naidoo, AI’s secretary general, said: “I have heard the concerns raised by some activists about allegations of discrimination at Amnesty India and have assured them that this issue is being treated with the seriousness it deserves.”