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.

The UK government gave Northern Ireland the power to reduce the level of the tax because it said it had a special economic case.
That prompted calls from Scottish and Welsh airports for similar treatment.
The UK Treasury said it had not ruled out devolving APD to Scotland and Wales in the future.
A report published following a consultation on reform of APD said the government at Westminster would "continue to explore the feasibility and likely effects of devolution to Scotland and Wales".
Earlier this year Chancellor George Osborne announced the duty would be cut for direct long-haul routes from Northern Ireland airports, and he would devolve control over the tax to the Northern Irish Assembly.
The move came in response to fears that some routes could be scrapped due to competition from Irish airports where APD is much lower.
Commenting on the UK Government's decision not to grant the Scottish government similar powers for now, Jim O'Sullivan, managing director of Edinburgh Airport, said: "APD is already costing Scotland passengers and having an impact on tourism revenues.
"We know from discussions with our airline partners that it is a major factor in their decision to connect further routes to Scotland.
"We would urge the Westminster Government to see Scotland as it does Northern Ireland and understand the need to both reduce and devolve this unfair and damaging tax."
Amanda McMillan, managing director of Glasgow Airport, said: "On the question of devolution of APD, Glasgow Airport has always been supportive of this proposal given the Scottish government's more progressive approach to aviation and its greater appreciation of the role the industry plays in supporting the growth of the Scottish economy."
Scottish Transport Minister, Keith Brown said the case for devolving APD was "compelling" and had been backed by all four of Scotland's biggest airports, and recommended by the Calman Commission.
He added: "We need to be able to deal with the competitive and connectivity disadvantages that Scotland faces and if APD were devolved now we could provide the means to incentivise airlines to provide new direct international connections to Scotland, benefiting our aviation industry and our passengers and supporting the growth of the Scottish economy.
"The UK government needs to listen to the many voices in Scotland who clearly want to see full devolution of the policy on APD."
University lecturers are due to start a two-day strike over pay, amid warnings other staff could join the dispute.
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Scottish airport operators have said they are dismayed and disappointed that the control of air passenger duty (APD) will not be devolved to Scotland.