Thousands of Ministry of Defence (MoD) specialist workers are on strike for 24 hours as part of a pay dispute.
Picket lines are to be mounted outside MoD sites across the UK
Up to 6,000 engineers and scientists belonging to the Prospect union are among those on picket lines outside military sites nationwide.
The union said the below inflation pay rise of 2.5% being imposed on staff was "just not good enough".
The MoD said the two-year deal was worth an average of 3.5% because some staff would also receive bonuses.
The department said the overall offer could not be improved.
Picket lines have been mounted outside military establishments across the UK on Wednesday, including the MoD headquarters in London, Devonport dock yard in Plymouth, Portsmouth Naval Base and Faslane Naval Base on the west coast of Scotland.
Prospect said ministers should value skilled staff.
Some 65 operational workers in sensitive jobs have been exempted from taking industrial action, but up to 6,000 members are still expected to stop work.
Mr Jary, the union's national officer, said: "For years our members have worked at immense personal cost to deliver the vital support without which the front line cannot function.
"Now the MoD wants to reward them with a below inflation pay offer stretching across two years. That is just not good enough."
Nearly 250 staff at Devonport Naval Base in Plymouth took part in the strike action, but the MoD said safety was not being compromised at the dockyard.
Prospect spokesman Nick Wadge said the major impact was that there had been no military movements of ships in the port of Plymouth on Wednesday.
The union said about 180 staff from Faslane supported the strike, a figure the base put at 114.
In January thousands of civil servants went on strike across the UK, in a row over job cuts and pay.