The three-day strike kicked off on Wednesday, with organisers threatening further action if a deal cannot be reached.
The University and College Union (UCU) warned more than a million students and 58 institutions would be affected by the action.
Picket lines were expected at campuses across the country, as well as other forms of industrial action - including staff strictly working to contract and refusing to carry out any additional duties.
On the first day of the strike, students also stood outside the head offices to Universities UK (UUK), holding banners saying “free and liberated education” and “UniNot4Profit”.
At the University of Manchester, students have also occupied a building in solidarity with staff on strike.
The UCU said it launched strike action after university bosses refused to withdraw pension cuts and to address falling pay, as well as “worsening” work conditions.
The union has suggested staff pay has dropped by around a fifth after 12 years of below inflation pay offers. It also accused universities of refusing to engage in action plans to address “widespread casualisation” and “excessive workloads”.
Jo Grady, the UCU general secretary, said: “It is deeply regrettable that staff have been forced into taking industrial action again, but sadly university bosses have shown little interest in negotiating in good faith and addressing the serious concerns of staff over falling pay, massive pension cuts, equality pay gaps and the rampant use of insecure contracts.”
She said staff were asking “for the bare minimum in a sector awash with money”.
The union boss added: “Sadly, the only time vice chancellors seem to listen is when staff take action, and those leading our universities should not underestimate their determination to change this sector for the better.”
The union is demanding that cuts to the pensions scheme are revoked and members are offered a £2,500 pay increase, as well as action to tackle “unmanageable workloads” and “insecure contracts”.
UUK - which represents more than 100 universities - said the industrial action over pensions was “deeply frustrating”.
“Despite a great deal of constructive work between employers, the USS Trustee and UCU, a small minority of staff seem determined to strike in protest at economic conditions they do not like, and a regulatory regime that universities are powerless to change,” the organisation said.
Nicola Dandridge from the Office for Students (OfS) said England’s higher education regulator was “extremely concerned” over the potential impact of the strike on students.
“Students have endured an exceptionally difficult time,” the OfS chief executive said. “It cannot be right that they face further disruption, and we would urge the employers and trade unions to work quickly so that any industrial dispute does not materially affect students. “
She added: “Universities are subject to consumer protection law, and they should consider how they will make up for any disruption caused by industrial action. This might include rescheduling any teaching which is missed, delivering course topics in a different way or considering whether partial refunds of tuition fees are appropriate.”
UUK said universities were “well-prepared to mitigate the impact of any industrial action” for students, which could include replacement teaching and resources.
A University of Manchester spokesperson said a “small number” had occupied a building on campus overnight. The students were not in the building - which was operating as usual - on Wednesday afternoon, they added.
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