Environmental protesters disrupted work at an open-cast coal mine in Wales on Tuesday, chaining themselves to machinery and blockading the entrance to the site.

Miller Argent, which operates the Ffos-y-Fran mine near Merthyr Tydfil, said production was not halted but two protesters were arrested. Others tried to unveil a banner reading “Coal: the black hole in UK climate policy” but were hampered by strong winds.

The protest highlighted growing controversy over the future of using coal for electricity generation. Eon’s plans for a new coal-fired power station at Kingsnorth have been condemned by Greenpeace and other campaigning groups, and the proposed site will be targeted this summer by the Camp for Climate Action.

Supporters of coal-fired generation say technology to capture and store carbon dioxide will allow coal to be used in a more environmentally sound fashion in the future, but green groups say this is years away.

Stephen Tillman, director of Miller Argent, criticised Tuesday’s protest on safety grounds. “The protesters have once again potentially put lives at risk, and not just their own. Because this is a site of former quarries and coal workings we are uncovering former deep shafts and trespass, potentially encouraging others including local young people to do the same, is not just irresponsible behaviour but a criminal offence.”

Coal from the Ffos-y-fran site is sent by train to Aberthaw power station, planned as a pilot site for carbon capture and storage technology.