Indonesian mining company Bumi, founded by financier Nat Rothschild, was on Thursday accused of human rights abuses at a heated annual general meeting.

Campaigning shareholders raised concerns over the alleged dumping of coal and chemicals into coral reefs, the mistreatment of workers and the company's lack of transparency.

But they were left disappointed after the board, which had co-chairman and biggest shareholder Indra Bakrie missing from the meeting, accused the representatives of "grandstanding" and "ranting".

After the meeting the campaigners, who go to around 20 mining AGMs a year, said Bumi's was one of the most poorly organised and least transparent they have attended. It lasted just 42 minutes.

Rothschild recently called on the company's chief executive, Ari Hudaya, for a "radical clean up" of the firm's complex ownership structure which involves some of the biggest businessmen in Indonesia.

During the AGM in London – the first time the FTSE-listed firm has held the shareholder meeting in the UK – Roger Moody from Nostromo Research was accused of "ranting" by the company's senior non-executive, Sir Julian Horn-Smith.

Moody raised concerns over the company's transparency but, in heated exchanges, he was told by Horn-Smith: "I will not be lectured to about human rights but will hear specific substantive points that you feel should be investigated.

"These are important questions, by the way, so don't misunderstand me because I'm not being dismissive, but let's do it in a way where we can engage and do something about it and address the points, rather than grandstanding."

He added: "I think you have come here with a policy of not wishing to engage with us. If you wish to engage with me I am happy to engage with you and any other shareholder who has a matter of such seriousness. We are not here to have a debate about human rights that will go on for several hours."

Chairman Samin Tan added: "The transparency is exactly why we are here. Things cannot be changed overnight."

Andrew Hickman, from Down to Earth, raised concerns over recent reports of coal and chemicals being dumped in a coral reef in Indonesia by the company.

He said: "This happened at various times and we'd like to know what is the credibility of the company if that is allowed to happen?"

Graciela Romero from War on Want added: "I understand from March 2011 there were workers who were mistreated after striking in order to have collective bargaining."

But the board said it was the first they had heard of the allegations and therefore could not respond immediately and would be happy to meet to discuss them in private.

After the meeting Hickman said: "I am quite shocked. The fact Bakrie doesn't take the time to come here is disgraceful. You'd have thought he would have faced up to the questions."

Richard Solley, from the London Mining Network,said: "The AGM was unique for its level of discourtesy and obfuscation. I've been to many mining AGMs and the thought that they would not have an answer to our questions is ridiculous. Bumi's refusal is unacceptable.

To suggest they don't know about the issues we raised, despite press reports and mentions in parliament, is ridiculous."

The company, which floated in 2010, had started trading at £10 a share and raised $1.1bn. On Thursday it closed at £3.15.

It was set up through a cash shell launched by Rothschild to fund acquisitions in the mining industry, but has been criticised for its complicated structure which involved the Bakrie family.

It led to Rothschild writing to the chief executive last year calling for reforms of its balance sheet.

But rather than take the advice, the board instead attempted to oust Rothschild as co-chairman. In the end the financier stepped down as co-chair, but maintains a position on the board.

At Thursday's meeting he sat at the far end of the long table laid out for the board at the Insititute of Directors and afterwards declined to discuss his previous misgivings. He said: "It would be inappropriate for me to comment."

A spokesman for company said: "We listened to the points raised and will be happy to engage." He added that Bakrie was unable to attend the proceedings because of other business commitments in the Far East.