"We've seen years of brutal austerity and vicious attacks on public services, overseen by an uncaring government whose pay policies are causing real hardship", he said.
Mr Serwotka said PCS members had endured a 10% pay cut because of the cap, one that will rise to 20% by 2020 if it is not lifted.
Delegates also disclosed that three public sector unions were planning to canvass members over coordinated strike action.
The Royal College of Nursing (RCN) is disappointed that the pay cap hasn't been removed with immediate effect and call for nurses to keep fighting until next years pay review body to recommend more than a 1 per cent rise.
Rehana Azam, national officer of the GMB said: "We do not welcome this announcement, which is nothing but smoke and mirror politics that insults our public sector workers".
"We need to make sure pay is fair for those in the NHS but we need to make sure it is fair for the tax payers that fund those services".
The Unison general secretary, Dave Prentis, also spoke in favour of the 5% demand. And a majority of our members will actually get a massive pay rise of 1.3%.
"There is a crisis in public sector pay and now is the time for action".
A final announcement on next year's pay deals will be made in the Budget in November.
"The Prison Officers" Association said it was considering strikes over what amounted to a real-terms cut, while TUC general secretary Frances O'Grady called increases for police and prison officers "pathetic".
Meanwhile, police chiefs warned that the pay award would put financial pressure on forces' already-stretched budgets and could impact on their ability to deliver services and avoid job cuts.
Chief Secretary to the Treasury, Liz Truss, said review bodies and departments were being given greater leeway to use pay to address "pinch points" within public sector staffing.
But Labour leader Jeremy Corbyn said he would scrap the cap completely "for all workers", adding: "The Labour Party totally rejects the Tories" attempt to divide and rule, to play one sector off against another'.
She said: "What we are making sure is that we look at it on a workforce-by-workforce basis because there are very different issues for teachers than for nurses and for police officers".
Mrs May's spokesman said ministers meeting in Cabinet at Downing Street on Tuesday "agreed that public sector workers are among the most talented and hard-working people in our society".
"There will still be a need for pay discipline over coming years to ensure the affordability of public services and the sustainability of public sector employment. They, like everyone else, deserve to have fulfilling jobs that are properly rewarded".