The treatment Orkambi could give patients an almost normal life expectancy if they start taking it when they are young enough.
While a deal was reached in May to make it available in the Republic of Ireland, the drug has not yet been approved for use by the NHS in any part of the UK.
The Cystic Fibrosis Trust said that of the 10,800 people in the UK suffering from the disease, 3,296 could benefit from the drug – including more than 240 patients in Scotland.
James Barrow, head of external affairs at the charity, said: “Cystic fibrosis is a cruel and life-shortening condition that places a huge daily burden on people living with the disease.
“Orkambi is a major step forward in the treatment of cystic fibrosis, which could help people live longer and has the potential to save the lives of thousands in the UK.
“The drug deals with the underlying cause of cystic fibrosis and prevents irreversible lung damage, which is especially important for children as they stand to benefit from it the most.
“If a deal between the government and the manufacturer over the price of Orkambi is not reached soon, we fear people in desperate need of the drug will run out of time.”
As well as the protest at Holyrood, similar demonstrations were to be held outside Downing Street and at the Welsh and Northern Ireland assemblies.Liam McHugh, from Castlederg in Northern Ireland, whose daughter has cystic fibrosis, was planning to attend one of the demonstrations.
He said: “My daughter is one of the few people in the UK lucky enough to get Orkambi as part of a clinical trial. For Rachel it has been nothing short of a wonder drug.
“When Rachel was born we were told she wouldn’t make it to adulthood. She is now 25 and, because of Orkambi, her lungs are better than ever.
“Looking at the strong and determined medical student she has become today, I’m the proudest father on the planet.
“I wish everyone dealing with this devastating disease could have the same chance of life.”