Members of global environmental movement Extinction Rebellion targeted Barclays bank in Market Street at around 1pm.
According to an eyewitness, activists in white overalls began applying paint to the building whilst unfurling banners across the pavement.
The group was raising awareness of climate change and Barclays' alleged support of the fossil fuel industry.
The paint is believed to be eco-friendly and protesters could be seen washing it off the brickwork before leaving.
At 2.30pm, activists left the bank and all paint has been removed from the bank's walls.
Malcolm Allen, chairman of Chorley Traders’ Alliance, said: "As far as I know - it was market day - and it didn’t affect trade in Chorley.
"Our biggest concern is if they targeted the precincted area - that would cause us problems.
"The first I knew of it was when it came over the town centre radio, that there were people in white outfits heading towards Barclay’s Banks, and that was it.
"It was only when I looked online they were targeting a few Barclay’s Banks regarding their investments in fossil fuels.
"It’s something we’ll be keeping an eye on.”
Extinction Rebellion has been approached for comment.