Workers at the Scottish Qualifications Authority (SQA) have gone on strike after rejecting a new pay deal.
Unions claim up to 55,000 exam appeals could be affected by the industrial action on Thursday, which is due to continue throughout September.
Unite described the SQA's latest pay offer as a "cynical publicity stunt".
But the exams body said staff on the lowest grades would benefit the most and warned the industrial action was "not in the interests of learners".
The strike at the exams body will continue next week on 15 and 16 September.
Union officials have also announced six further dates in late September and early October, as well as an overtime ban.
