David Cameron said Wednesday's strike by public sector workers was proving "a damp squib" with many key services continuing to operate.
Mr Cameron clashed with Ed Miliband over the issue at Prime Minister's Questions, accusing Labour of "taking the side of the unions".
But Mr Miliband accused the PM of "demonising" low-paid workers and "spoiling for a fight".
Unions said up to two million workers took part in industrial action.
The government says its figures show turnout was "much lower than these claims".
Thousands of schools have closed as a result and hospital operations were cancelled while courts and government offices were among other disrupted services.
But thanking those who had helped to keep public services going - including No 10 staff - Mr Cameron told MPs that early evidence suggested about 40% of schools were open, less than one in three civil servants had walked out while contingency measures at airports and ports were "minimising the impact".
There was also full ambulance cover and only 18 out of 900 job centres had closed, he added.
"It looks like something of a damp squib," he said.
Mr Miliband said the government was to blame for the strike, likely to be the largest by the public sector in years, by effectively calling time on negotiations and not meeting with the unions for nearly a month.
"What has the prime minister gone around saying? He has gone round saying he is privately delighted the unions have walked into his trap. That is the reality. He has been spoiling for this fight."
Despite government claims that low-paid workers would be protected, he said more than 800,000 part-time workers on salaries of less than £15,000 faced a "3% tax rise" on their pensions.
"The reason people have lost faith is he is not being straight with people," he said.
He added: "The difference is, unlike him, I am not going to demonise the dinner lady, the cleaner and the nurse."
Mr Cameron said he did not want to see strikes, and that negotiations were continuing. While the Labour leader had previously said it would be wrong to strike while talks were under way he had now changed his mind.
During stormy exchanges at PM's questions he accused Mr Miliband of being "left-wing, weak and irresponsible" and said Labour was unwilling to condemn the strikes because they were "in the pockets" of the unions.
"The leader of the Labour Party has taken sides today," he said.
"He is on the side of the trade union leader that wants strikes not negotiations, on the side of the people who want to disrupt our schools, disrupt our borders, disrupt our country."
Changes to public sector pensions were "absolutely essential", Mr Cameron added, and the offer made by the government was "very reasonable and very fair".
Amid a row over whether meaningful negotiations were going on, the Cabinet Office accepted that the last official talks between ministers and union leaders on the pension issue had taken place on 2 November.
But it said talks on a scheme-by-scheme level with civil servant and local government representatives had taken place since, while meetings with teaching and health unions were scheduled in the next few days.
Later the government said less than a third of civil servants had taken part in the strike, only 14 out of 900 job centres and benefit offices were shut and there had been "no serious disruption" at airports and ports.
Ministers said only 79,000 health workers were on strike in England, while Unison said more than 400,000 NHS workers across the UK took part.
It is thought that about 62% of state schools in England were closed, 66% in Northern Ireland, 86% in Wales and more than 98% in Scotland.
Unison, which represents 1.3 million workers, said the prime minister was sounding "increasingly desperate".
"I wouldn't call two million people taking strike action a damp squib," said its general secretary Dave Prentis.
"He has only to turn on the TV, or listen to the radio, or look out the window, to see nurses, dinner ladies, paramedics, social workers, teaching assistants, lollipop ladies amongst others standing up for their pensions.
"And the thousands of picket lines, demonstrations, rallies and events are not a figment of our imagination. These people are angry public servants who the government has driven to the end of their tether."