North Sea workers are said to be "solidly" supporting a 48-hour strike in a bitter dispute over pay and conditions.
Members of Unite and the Rail, Maritime and Transport union (RMT) employed by the Wood Group walked out at 6.30am saying they face a "massive" cut in pay and allowances of up to 30%.
Unions accused the company of turning down an offer to suspend the strike if the proposals were removed, to allow for further negotiations.
The company denied the claims.
RMT general secretary Mick Cash said: "The action on the Wood Group Shell assets is solid as a rock again this morning.
"After savage redundancies and attacks on workload and working conditions this group of offshore workers have been told that they are going to be railroaded into accepting pay cuts of up to 30%.
"They have chosen not to bend the knee, they have chosen to stand up and fight.
"The unions welcome the fantastic support that has been pouring in from around the world for this important dispute. We are also well aware that the company chief executive has had a pay increase of 28% to bring him up to £600,000.
"It is obscene that while the top bosses are lining their own pockets they are kicking the workforce from pillar to post."
Dave Stewart, chief executive for Wood Group's eastern region business unit, said: "We are hugely disappointed that industrial action is progressing despite the significant movements made and what we believed was constructive, continued engagement with the unions over the weekend and on Monday. "
"We had agreed to suspend the implementation of the terms and conditions currently proposed to enable further detailed discussions with our employees and the unions. We believed the unions had also agreed to this during our meeting on Monday.
"We were extremely disappointed by the last minute change to what we feel was a very constructive way forward and are actively seeking clarity on why this decision was taken."
Mr Cash and other RMT officials staged a protest outside Shell's head office in London in support of the strikers, who work on Shell platforms in the North Sea.