It was the 11th walk-out of the UK-wide dispute which shows no signs of stopping.
Tomorrow will see Crown branches – the bigger, high-street branches – in England, Wales and Northern Ireland follow suit, after their bank holidays.
The CWU and Post Office Ltd are locked in a dispute over pay and restructuring.
Bosses want to franchise 70 of the Crown post offices so they are run through a retailer, affecting the future of 4,000 workers.
The CWU opposes the franchise plan and is also demanding a 3.5 per cent pay increase in 2012-13, and a further 3.25 per cent rise in 2013-14, which the Post Office has rejected.
Post Office bosses say the 370 Crown businesses – as opposed to the smaller sub post offices – are losing £40 million a year of public money.
Today’s strike action did not delay post, but did stop customers using the many services larger branches offer, such as tax forms, currency exchanges, driving licences and passports.
The ten branches to close included Duke Street, Hecla Avenue and Springburn Way, all in Glasgow; Edinburgh’s Wester Hailes; and Union Street in Aberdeen.
Alloa, Cumbernauld, Kirkwall, East Kilbride and Kilmarnock were also closed.
Of the remaining ten, nine were open and one was closed because of a local bank holiday.
Kevin Gilliland, network and sales director at the Post Office, said: “We regret any inconvenience caused to customers by today’s strike action.
“However, almost half of our Crown branches in Scotland are open and the remainder of our sub post offices are unaffected.
“We are extremely disappointed at the CWU’s decision to call further strike action.
“This action can only cause disruption to customers, cost our people money and place further pressure on the Crown network which is currently losing £37m a year.
“We must continue with our plans to turn around the Crown network to ensure we keep these branches on high streets and in city centres across the UK.
“We remain open to discussions with the CWU on pay options which do not add to the current loss of public money.”
The CWU warned the Post Office would have to change its position if it hopes to avoid further industrial action.
“It’s an ongoing dispute – this is the 11th day of action and it could continue,” a spokesman said. “This is just a taste of things to come unless the Post Office comes back to the table.
“Feelings are very strong. People have endured 11 separate days of industrial action across three months. That’s how strongly they feel about this.”
The union also rejected the Post Office’s claims that the changes are being forced by financial pressures.
Deputy general secretary Dave Ward said ahead of Saturday’s strike: “The message can’t be much stronger to Post Office management. Crown post office workers do not agree with management’s slash-and-burn approach and are prepared to take prolonged industrial action to defend jobs and services and win a fair pay rise.
“This is a company which made £94m profit last year and paid out £15.4m in bonuses to senior managers.”