Protesters on a roof at the Ffos-y-Fran site
Police have arrested two people after demonstrators chained themselves to excavation machinery at the site of an opencast coal mine near Merthyr Tydfil.
There were thought to be almost 40 protesters at the Ffos-y-Fran site, stopping work and blocking one of the site entrances from 0630 GMT.
But South Wales Police said most of the demonstrators left the site of their own accord at around 1600 GMT.
Developer Miller Argent condemned the "irresponsible behaviour".
The protesters left the site by early evening, saying their aim had been to draw attention to the UK government's policy on climate change.
Earlier, 15 protestors were inside the site and a further five were on the roof of a site building.
Another 10 were on a digger, four were chained to a gate at Cwmbargoed disposable point and four were lying across the road.
The demonstrators said they were highlighting the "hypocrisy" of government claims it is tackling climate change.
Residents have already protested about the mine which will become one of Europe's biggest opencast sites.
Protesters outside the Ffos-y-Fran site
In an e-mail sent to the BBC, the protesters said they intended to remain on the site for "several days".
They said coal had the biggest impact on climate change of any fuel and that the government was "supporting an outdated and dangerous technology that has no future".
Residents have claimed there will not be enough protection from noise, dust and pollution from the site and AMs debated plans for a 500m (1,640ft) buffer zone between houses and new opencast mining developments last month.
There has been anger that the opencast site had been allowed to go ahead with a buffer of just 40m (130ft) to the nearest homes.
Opponents of the scheme previously halted preparation work at the site in December by "occupying" two large excavating machines.
Developer Miller Argent (South Wales) said as well as the break-in and seizure of heavy plant, police had also been told of an incident of suspected sabotage to a water treatment area last night.
Stephen Tillman, of Miller Argent, said: "Our primary concern is for the safety of our workforce on site. 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."
Miller Argent said it had called on police for assistance and was considering what further actions to take.
"We are extremely disappointed on behalf of our workforce and the people of Merthyr who want this reclamation to be completed, that protesters have tried to disrupt work once again."
Ffos-y-Fran will mine 10m tonnes of coal over 17 years.
Friends of the Earth Cymru said residents had been "badly let down" by the planning process and "peaceful protest is an inevitable consequence."