Staff at 15 universities across Scotland are taking part in strike action.
The University College Union (UCU) and the Educational Institute of Scotland (EIS) University Lecturers Association (EIS-ULA) called the strikes in disputes over pay and conditions.
The UCU disputes - on pay, working conditions and pensions - are UK-wide and will take place across 14 days.
The EIS-ULA dispute is over pay only and will happen across five days.
The UCU strikes are affecting 74 universities across the UK, the largest number since a two-day, nationwide demonstration in 2016.
It will be the second wave of strikes after a related eight day demonstration just before Christmas.
UCU Scotland official Mary Senior said: "This unprecedented level of action shows just how angry staff are at their universities' refusal to negotiate properly with us."
Meanwhile, the EIS said its strike was over lecturers' pay which it said had been "cut, in real terms, by at least 20% [over] the past decade".
Its demonstrations will spread out across five days until 13 March.
Larry Flanagan, general secretary of the EIS said lecturers in Scotland were "poorly served" by UK management negotiations.
"Our members have been forced into this action because university management refuse to negotiate a fair and reasonable offer for lecturers," he said.
"University lecturers have endured a decade of declining pay and soaring workload, and have been offered a meagre, 1.8% pay settlement this year - well below the recent settlements in the school and college sectors, here in Scotland, and, indeed, below the average public-sector settlements over the past year."
About 600 union members from five colleges are involved in the EIS-ULA walkouts.
In response to the action, a spokesman for Universities and Colleges Employers Association (UCEA), the body representing the universities said: "Universities are deeply disappointed to see UCU trying to press ahead with their HE committee's plans for extensive strike action.
"UCEA has offered UCU further informal talks and urges the union's leaders to reconsider pursuing damaging strike action at less than half of universities, damaging students, staff and their own members - who are yet to be consulted over the new positive proposals that are on the table.
"These proposals address the important issues around employment in universities, focusing on casual employment, workload/mental health and gender pay gaps/ethnicity pay."
The EIS previously said such proposals were offered for these issues but there had been no scope for negotiating pay.
Universities involved in industrial action:
The Open University in Scotland
University of Dundee
University of Stirling
University of Edinburgh
University of Glasgow
University of St Andrews
University of Strathclyde
University of Aberdeen
Glasgow Caledonian University
Glasgow School of Art
Queen Margaret University
The Scottish Association for Marine Science
University of West Scotland
Edinburgh Napier University