Nearly half of West Dunbartonshire pupils had their results downgraded, according to the area's MSP.
Jackie Baillie MSP said she had spoken to a senior education officer at the local authority who confirmed 43.8 percent of Higher pupils and 43.3 percent of Advanced Highers students had their grades reduced.
She told the Lennox Herald the figures were "shocking".
It came as students protested against the downgradings at a rally in George Square in Galsgow today.
Teenagers received their marks on Tuesday which were calculated in a different way after exams were scrapped during lockdown.
While some are delighted to have achieved the grades to get into university, others have been left bitterly disappointed after seeing a drop in results.
A quarter of exam entries were downgraded in Scotland after estimates by teachers were adjusted by SQA officials.
The Dumbarton MSP wrote on Twitter: "In West Dunbartonshire 43.8% of Higher pupils had their results downgraded by the @SQA and for Advanced Highers it was 43.3%. Teachers estimates of individual achievement ignored and a blanket approach by @SQA that lowers grades for those from working class areas #SQAshambles
She told the Lennox Herald: "These figures are shocking.
"I felt really depressed for all the young people affected when I heard these figures.
"They represent our future and yet they have been treated so badly.
"The blatant approach taken is shocking and our young people deserve so much better."
"Teachers put a huge amount of work into establishing the grades for students. They looked at all their classwork and exams and discussed it as a team but the SQA has ignored all of that."
West of Scotland Green MSP Ross Greer called for the SQA to adopt a ‘no detriment’ policy for young people in West Dunbartonshire who appeal their grades, guaranteeing that grades are raised to at least the level achieved by that young person in their prelim exams.
He said: “I have been contacted by senior staff at schools who have seen over 90 percent of their Higher and Advanced Higher grades changed, almost all lowered and in every one of these cases the school has been in a deprived community.
“I’ve also had messages from young people who cannot understand why, despite receiving strong A grades in their prelims, they were awarded a C or even D by the SQA.”
An SQA spokesman said: " We believe we have delivered fairness to learners, through a consistent, evidence-based approach in the absence of exams.
"We have maintained the integrity and credibility of our qualifications system, ensuring that standards are maintained over time, in the interests of learners, through judicious moderation of grades.
“The most disadvantaged young people have achieved better results in 2020 compared to both 2019 and the average results for the last four years. At grades A to C, the attainment gap between the most and least disadvantaged young people is also narrower this year for National 5, Higher and Advanced Higher than for last year or the average gap for the last four years.”
“We know from experience that not all learners sit prelims. A prelim does not represent the whole course assessment, nor does it cover the whole course content. The vast majority of SQA’s National Courses have multiple components with a final exam being just one element.”
A free appeals service opened on Tuesday with priority given to those wishing to confirm a college or university place.
