Staff at more than 40 post offices in the UK are holding a 24 hour strike in a row over plans by the Royal Mail to close branches.
Many "Crown" Post Offices have been earmarked for closure
The Communication Workers Union (CWU) said up to 500 members had walked out in the "very well supported" action.
But the Royal Mail said all of the 42 affected post offices in the north and west of the capital were unaffected and remained open for business.
The CWU said the closure programme had led to hundreds of job losses.
The shut-downs affect the so-called Crown Post Offices, which are directly run by the Royal Mail, some of which are being axed as part of a major shake-up.
Andy Furey, national official of the CWU, said the strike aimed to bring the two sides to the bargaining table.
He added that the Post Office had bussed in managers "at great expense" to keep the strike -hit branches open.
"Closures also mean inferior services for local people and will add to the degradation of the Post Office network," Mr Furey added.
However, a Post Office spokesman denied the claims, saying most branches are successfully managed through franchise agreements with subpostmasters or retailers.
"Franchising has been a proven model for many years as one of the ways in which we can reduce costs, whilst bringing benefits to customers with longer opening hours and better surroundings," the spokesman added.