A protest by climate activists halted work at the UK's largest opencast mine in Merthyr Tydfil on Tuesday.
Campaign group Reclaim the Power said it hoped to stop operations at the Ffos-y-Fran site - two days before the assembly election.
About 300 protesters were joined by Green Party leader Natalie Bennett, with the majority leaving by 17:00 BST.
Miller Argent, which runs the site, said it supports 230 jobs and affordable electricity.
South Wales Police said there was "nothing to suggest a risk of disorder".
"However, we do have a visible police presence around the area and sufficient police resources on standby to manage any public safety issues," Supt Phil Ashby said.
The mine has coal reserves of 11 million tonnes.
Caerphilly council rejected Miller Argent's application for a new 478-hectare (1,180-acre) opencast mine at Nant Llesg near Rhymney in August 2015. The company is appealing against the decision.
Neil Brown, managing director of Miller Argent South Wales, said: "It's a local industry, it's Welsh jobs, people don't realise we support the steel industry and we support affordable generation."
The protesters want a moratorium on Welsh opencast mining, saying a suspension would prevent the pollution of homes and the environment.
Ellen Gibson, from Reclaim the Power, said: "Now we are joining them to resist coal company Miller Argent's attempts to override local democracy and force through a new mine next door."
Welsh Green Party leader Alice Hooker-Stroud has also joined the protest.
She said: "Fossil fuels must stay in the ground if we're to act responsibly on climate change. We don't need or want more opencast in Wales."
Steffan Messenger, BBC Wales environment correspondent
Early this morning, a small group entered the site and chained themselves to heavy machinery.
At 07:30 BST they were followed by four large groups of campaigners who entered the mine from different locations.
They are dressed in red as a reference to the demonstrations which took place outside the UN climate conference in Paris last year.
It is to symbolise the red lines they believe should not be crossed on the use of fossil fuels.
The protesters say they intended to occupy the mine for several hours.
Work at Ffos-y-Fran has come to a halt. There is a large police presence and a police helicopter is circulating.
Outside the main gate there is a group of 50 protesters staging what they are calling a "family friendly" demonstration.
Miller Argent, which operates the site, says it is concerned about the safety of all involved.
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