Opponents of an opencast mining scheme in Merthyr Tydfil halted work at the site by "occupying" two large excavating machines.
Work was stopped after protesters took over two machines. Photo: Greenpeace
Several campaigners, some dressed as polar bears and clowns, had staged the protest, which ended at about 1615 GMT.
Ffos-y-Fran will become one of Europe's biggest opencast sites, mining 10m tonnes of coal over 17 years.
Developer Miller Argent (South Wales) has said the site will reclaim derelict land and bring benefits to the area.
But local people claim the site is just 36m (118ft) away from some houses, raising concerns over noise and dust.
Protestors left the site on Wednesday afternoon and were questioned by police, although no arrests were made.
Environmental groups are opposed to the possible effects of burning huge quantities of coal.
Miller Argent (South Wales) Ltd said it was already bringing benefits to the area and would continue to do so.
The company has been asked for a response to the protests.
Demonstrators arrived on site and took over two large machines.
Environmental campaigner George Monbiot said: "We are here at the invitation of the people of Merthyr Tydfil who are confronting an appalling human rights abuse - that a vast opencast pit, the Ffos-Y-Fran pit, is being built within 36 metres of the nearest home.
"It's going to be 200m deep and it is going to have a horrendous impact on their lives.
"But we are also here because we are concerned about the impact on climate change of digging all this coal out of the ground.
"There's going to be 11m tonnes of coal dug out here which will create 30m tonnes of carbon dioxide when it is burnt.
Work was stopped after protesters took over two machines
"That is equivalent of the sustainable emissions of 55m people for one year.
"It is simply unacceptable when we are trying to prevent runaway climate change from taking place."
A large police presence was drafted in to monitor the protests at Ffos-Y-Fran.
Mr Monbiot said several of the protesters were dressed as polar bears to highlight the loss of eco-systems across the world and others were dressed as clowns in an attempt to prevent tension.
He added: "We have occupied the two big excavators.
"They are two huge machines and they are what feed all the other machines, they dig out the dirt and coal which gets taken away by other trucks.
"By occupying the big excavators we ensure no digging can take place today."
The developers have been carrying out preparation work to open the site.
The firm initially received planning permission following a public inquiry, but it was overruled by the High Court.
The Welsh Assembly Government, which backed Ffos-y-Fran, won the right to appeal and a Court of Appeal judge allowed it to go ahead.