Protestors opposed to the franchising of more than 70 Post Office branches to retailers have staged demonstrations in high streets across the UK in an attempt to save the “great British institution” from closures and sell-offs.

Union officials queried the wisdom of closing crown post offices, those directly managed by Post Office Ltd, since the company is profit-making.

They added that franchising causes people to leave the service because jobs advertised by firms such as WH Smith, which holds a number of franchises, are lower paid than those at the Post Office.

“People are sick and tired of the broken privatisation agenda that is behind this latest move,” said Dave Ward, the general secretary of the Communication Workers Union (CWU). “We hope many will join our campaign to save our post offices.”

About 50 demonstrations attended by Labour politicians and hundreds of local campaigners took place on Saturday in every region of the UK, from Truro in south-west England to Aberdeen in north-east Scotland, in what unions called “a day of action” aimed at publicising “this latest shameful round of closures”.

Thousands of signatures were gathered to demand that the future of the Post Office is debated in parliament, amid claims that it could follow a similar course to the now privatised Royal Mail. Labour has pledged to renationalise the industry.

The Post Office remains state-owned, having been split off from the privatised Royal Mail in 2013. The government was accused of short-changing taxpayers after shares in the privatised postal service rose by 38% within a day of flotation.

“We’ve had fantastic support today,” said Vinnie Micallef, the CWU’s east London branch secretary.

“These post offices are profit-making. Post Office Ltd say the closures are part of a cost-cutting drive, while paying themselves handsomely and destroying postal workers’ contracts and pensions.”

Post Office profits almost tripled from £13m to £35m in 2017/18, with bank branch closures credited for driving increased performance within its financial services, telecoms and insurance arms.

A WH Smith spokesman said: “Post offices are a vital hub providing essential services to the community and we take the social responsibilities associated with this very seriously.”

Roger Gale, the Post Office’s network sales director, said: “The Post Office is committed to keeping its vital services on high streets and at the heart of communities across the UK.

“However, we are not immune to the challenges facing retailers in local high streets, and we must adapt to changing customer needs by making our services more accessible to customers, for instance through longer opening hours.

“That is why we are proposing to franchise a further 74 directly managed branches to WH Smith.”