Strike disrupting 8,000 UK schools
The walkout by thousands of teachers has closed or partially closed up to 8,000 schools, forcing working parents to stay at home or find childcare.
The National Union of Teachers (NUT) is staging more than 50 rallies, as members demand a 4.1% pay rise rather than the 2.45% on offer.
Gordon Brown has described the strike as "unfortunate and regrettable".
This one-day strike, condemned by all the main political parties, is the opening move in what the NUT says will be a long-term campaign over pay.
Christine Blower, acting general secretary of the NUT, said she could not rule out further pay strikes in the months ahead.
"It is not in my gift to give that guarantee," she told the BBC.
It was estimated that more than 2.5m pupils were affected by Thursday's strike - with more than a million forced to stay at home. Many are studying for GCSE examinations which start next month.
The prime minister said the strike had been called by a "minority" of teachers, and said it was regrettable for both pupils and parents.
Mr Brown added: "I hope we can move forward in the next few months and get a sure settlement of this."