BERLIN (AP) — Thousands of people are protesting against the expansion of a coal strip mine in western Germany that would entail the chopping down of an ancient forest.

Police in the city of Aachen estimated that 4,000 protesters took part in a march toward Hambach forest Sunday. Some carried saplings they planned to plant between the woodland and the adjacent lignite mine operated by German utility company RWE.

Authorities have been trying for days to evict environmentalists who chained themselves to treehouses and tree trunks.

German news agency dpa reported that two men who were hiding inside a makeshift tunnel left voluntarily after medics warned they could run out of oxygen.

Environmental groups hope a government-appointed committee examining the future of Germany's coal industry will recommend halting the destruction of Hambach forest.