Work at Ffos-y-Fran began in June 2007 after a public inquiry
Two men have been arrested on suspicion of aggravated trespass after a protest was held at an open cast mine site.
Workers at the Ffos-y-Fran site near Merthyr Tydfil were left idle after the incident on Wednesday.
Lorries at the Miller Argent owned site were stopped from transporting materials from going in and out of the site due to safety reasons.
South Wales Police said two men were later arrested for aggravated trespass and the road was reopened.
It is understood that the protest involved people being suspended in a net under a bridge.
In a statement, South Wales Police later said that officers at the scene had read out a criminal warning for aggravated trespassing.
It read: "This resulted in the two protestors deciding to end their protest and officers assisted in this to ensure their personal safety.
"The two protestors were then arrested for aggravated trespassing and taken to custody in Merthyr police station."
In August, campaigners set up climate change camp near the mine for a three-day protest
Work to mine 10m tonnes of coal over 17 years from Ffos-y-Fran began in June 2007 after a public inquiry.