The University and College Union says the 1.1% rise offered by the universities is "an insult".
But the Universities and Colleges Employers Association said the walkout was "disappointing given the very good pay offer".
Unions representing university support staff are balloting on the offer, with strike action possible in the autumn.
UCU says its members have suffered a real-terms pay cut of 14.% since 2009 and complains the squeeze on staff salaries has come as university leaders enjoyed hefty increases.
"A 1.1% pay offer is an insult to hardworking staff, especially in light of the 5% pay rise vice-chancellors have enjoyed while holding down staff pay," said general secretary Sally Hunt.
"Industrial action which impacts on students is never taken lightly, but members feel that they have been left with no alternative.
"If the employers wish to see a swift end to this dispute, and avoid further disruption, they need to come back to the table with a much-improved offer."
Summer exams are still running at some universities, though many have finished.
A spokesman for the employers anticipated only "minor impact and minimal student disruption".
"Even for examinations which are still taking place at some higher education institutions, more than nine out of 10 report that a no to low impact is anticipated," said the spokesman.
"We would like to see the UCU consulting its members on the final offer."
The employers say the offer is "at, and, for some, beyond, a limit of affordability for higher education institutions and the very best offer that will be available this year".
They maintain the weighting of the offer means the worst paid university staff will get a rise of more than 5%.
They say they have also offered talks on zero-hours contracts and on improving lower pay for female academics.
But UCU says it rejected the 1.1% offer as it was only a marginal improvement on the original 1% on which it had balloted members.
Ballots of university support staff represented by Unison and Unite are also under way on the improved offer, with both unions recommending it be rejected.
Any action would take place during the autumn term, said a Unison spokeswoman.
UCU is planning strike rallies in:
Staff are also working to contract from Wednesday - refusing to set extra work, cover for absent colleagues or work overtime.
University lecturers are due to start a two-day strike over pay, amid warnings other staff could join the dispute.