Two ambulances and a mountain rescue team were blocked by illegal parking on overcrowded roads around Loch Lomond and The Trossachs National Park.
The incidents happened near Rowardennan on East Loch Lomond on Saturday and at Callander Crags on Sunday.
The first hold-up came as emergency crews were trying to help someone in trouble on Ben Lomond.
A car owner has been charged after the second incident in which emergency access was cut off.
Police said there were also overcrowding and irresponsible parking issues near Ben A'an and Ben Venue in the Trossachs, and at Loch Ard near Aberfoyle.
Visitors to the national park are now being warned not to park irresponsibly or they face fines and having their vehicle towed away.
Insp Andy Bushell said: "There are designated parking areas within the national park. If these are all in use then vehicles must seek other permitted parking zones and not leave cars on narrow access roads or public walkways.
"Recent incidents of inappropriate parking have resulted in emergency service vehicles being unable to pass and this is totally unacceptable."
Simon Jones, director of visitor operations at the National Park, said irresponsible parking had become a particular issue since lockdown restrictions eased.
"These incidents and the many other examples of reckless parking on roadsides and crowded car parks highlight just how dangerous this behaviour is," he said.
"It is extremely lucky that the outcome in both of these cases was not a lot worse, but it could be next time."