Save Our Landscapes was set up to oppose proposals by global cement company Cemex to extend Hyndford Quarry onto land near New Lanark and the historic viewpoint overlooking the Falls of Clyde.
Now the group has received international backing from visitors to the area, who have flocked to sign a petition for the plans to be thrown out when they go before South Lanarkshire Council for approval.
Tourists from as far away as the United States, Guatemala and Australia have joined the group to call for the plans to be blocked, while more than 9000 local people have also raised objections.
The local authority has already received more than 2000 letters of objection from residents of every postcode in the area and letters from every Scottish parliamentary constituency.
The plans to extend the quarry are expected to go before South Lanarkshire Council on October 8.
Save Our Landscapes leader Mark Stephens said among those who had objected to the plans were 100 tourists from America, 50 from Australia, and more than 60 from Germany and France.
He said: "These are people who have visited the Falls of Clyde, and the petition reflects where the visitors to the area are coming from. This area attracts a wide mix of visitors, and while most are from the local area, many are from much further away.
"We have gained support from all over Scotland and internationally as well. What this shows is how important the area is as a resource for tourism and how much it contributes materially and economically as a destination."
New Lanark was established in 1785 as a cotton mill village and holds a special place in Scotland's cultural history after becoming the focus of Robert Owen's utopian socialism, which provided residents with free health care and affordable education.
The Falls of Clyde are surrounded by a nature reserve managed by the Scottish Wildlife Trust. It is home to more than one hundred species of birds, including a pair of nesting peregrine falcons.
Cemex's proposal to extend the quarry into the buffer zone has been opposed by some MSPs, with a group of 20 parliamentarians from across the political divide signing a motion for Scotland to protect its history.
International Council for Monuments and Sites vice-president James Simpson has warned the project risks undermining Scotland's reputation for safeguarding its heritage.
Cemex, headquartered in Mexico, has already come under pressure after it emerged UK and American environmental agencies took action against it more than 40 times.