The group has called for urgent action to be taken by authorities after an investigation revealed the "dreadful" state of banks alongside the A82.
The Friends of Loch Lomond and the Trossachs spent money opening up views of Loch Lomond from the road, which runs along the west of the attraction, but when members conducted a pre-Easter investigation it found rubbish strewn in lay-bys.
The warning comes after it emerged the Loch Lomond and The Trossachs National Park is to consider extending by-laws prohibiting alcohol and camping across the park, after the rule changes were credited with a drop in anti-social behaviour on the eastern banks of the loch.
James Fraser, chairman of the Friends of Loch Lomond and The Trossachs, said: "I was shocked to see the dreadful condition of a number of lay-bys at lochside beauty spots, which are currently a very poor advert for the National Park and Scotland's aspirations to be a world-class tourist destination.
"Extensive amounts of litter are strewn all over the lay-bys and surrounding areas - and this is even before the busy Easter holiday period and the main tourist season gets under way.
"In what is supposed to be a special year for Scotland welcoming millions of visitors from around the world, it is clear effective management of what should be a high-quality tourist route and visitor journey experience alongside the bonnie banks is low down [on the] list of priorities.
"I am calling for urgent action to introduce a more regular programme of litter clearance more fitting for one of Scotland's top tourist attractions, which is a magnet for four million visitors every year."
Argyll and Bute Council confirmed it had responsibility for litter clearance along the A82. A spokesman said litter was cleared on a weekly basis, all year round. He added: "It is a priority as the lay-bys are highly visible. However, we appeal to the public to take litter home with them."
The friends group has been lobbying for a comprehensive tourist route action plan to be drawn up and implemented for the A82 corridor between Balloch and Tarbet on Loch Lomondside. They claim the visitor experience falls well short of what is needed for what is considered one of the country's most beautiful places.
The introduction of by-laws in 2011 to the east of the loch, on a nine-mile stretch between Drymen and Rowardennan, has been credited with producing drops in reports of vandalism, violence, littering and fires.
A report to the Scottish Government on the operation of the legislation over the past three years, produced last month by National Park, says the new rules had contributed to a "radically improved visitor experience in the area" while boosting the local economy.
The National Park is set to embark on a review of "visitor management options" for other popular locations, with figures including Labour MSP Jackie Baillie saying not enough had been done to protect communities on the west of the loch from problems associated with a huge influx of tourists.