The parents have used their cars to blockade Keeton's Road, Bermondsey, to prevent contractors from driving heavy vehicles into a grass courtyard, between several blocks of flats, that is used as a play area.
The square at St Crispin's estate is due to be partly excavated so cement can be poured under flats to stabilise them against subsidence once construction of the new Tube tunnel directly underneath begins later this year.
However, about twenty mothers are obstructing the preparatory work, alleging children risk being injured
during the dig.
One, Ann Crispin, said the blockade would not end until London Underground agreed to provide an alternative children's play area. The protesters, who have hung banners from the flats, are also concerned that inadequate soundproofing measures are being introduced to shield them from construction work on the line during the next two years. The flats back on to the site of the planned Bermondsey Underground station.
Yesterday Mrs Crispin said: 'The dumper trucks were driving right past people's front doors, and drilling was being carried out a few feet from where children were playing.
'This is the start of the school holidays, and there's no way you can keep kids in all
the time. We can't let them play anywhere else because there are too many main roads around.
'London Underground has ridden roughshod over us, they have been using bullying tactics. We want a commitment from them in black and white that they will be more considerate and use the highest safety standards.
Richard Humphries, public relations manager for the Jubilee Line extension project, hoped an agreement would be reached today so work could continue. 'So far all we are doing is putting up hoardings to prepare the area for drilling. We are keen to proceed quickly and are asking the parents what they want to try and find a solution.
'Safety is a major concern, and we have said we will install protective barriers at entrances to buildings, while dumper trucks leaving or entering the site will have someone walking in front to warn pedestrians.
He added one option under consideration is postponing the work until after the summer.