Members of the EIS Further Education Lecturers' Association want to see more action taken to end differences in pay between colleges.
The union said lecturers had waited with "great patience" for colleges to honour a deal agreed a year ago.
Colleges Scotland said the action was "hugely disappointing" at a "critical time" for students.
The union has threatened to strike three days every week if the dispute is not settled.
Further proposed strike dates are:
EIS general secretary Larry Flanagan said: "Lecturers have shown great patience in waiting for colleges to honour the deal that was agreed last year.
"However, after more than a year of waiting and watching college managers talk down the agreement that they freely signed up to, Scotland's FE lecturers have simply had enough.
"All that lecturers are asking is for the deal that was agreed by both sides to be honoured."
Mr Flanagan also accused college management of attempting to "rewrite history" and "airbrush" a binding agreement.
Colleges Scotland Employers' Association said it was doing what it could to minimise disruption to students currently preparing for exams and submitting coursework.
A spokeswoman said: "The strike is completely unnecessary and inappropriate at a time when we are currently engaged in Acas talks with the EIS to try and resolve this dispute.
"Harmonisation of pay and conditions is a complex process that requires compromise, not strikes and disruption.
"We are, however, pleased that the EIS now recognises the agreement reached in March 2016 is a 'deal on pay and conditions', because that is the simple reality."
The organisation added that it had already agreed to an average pay rise of 9% over the next two years.
Scottish college lecturers have gone on strike in a dispute over pay and conditions.