London Underground bosses have contacted the conciliation service to help arrange fresh talks over the Tube pay dispute, which closed down the network today, causing travel misery for millions.
Acas was asked to assist to get the two sides back around the negotiating table to avoid a repeat of the 24-hour strike which ends this evening.
Tube stations were closed and other forms of transport were packed, even though Transport for London put on 200 extra buses.
Twice as many people as usual hired bikes and many commuters swapped office clothes for shorts and trainers as they walked to work in bright sunshine.
TfL said the bus network performed well, but traffic was heavy, and the rush hour was starting early for the second day in succession.
Business groups said the strike would cost the capital's economy tens of millions of pounds.
Picket lines were mounted outside Tube stations by members of four trade unions involved in the action in a row over the new all-night Tubes, due to start in mid-September.
Steve Griffiths, London Underground (LU) chief operating officer, said: "We thank Londoners for their patience today as we work hard to help them make their journeys.
"It's been very busy, particularly on the bus and road networks, although many people are travelling outside peak hours and walking and cycling.
"Our staff are working hard to help by providing maps, travel advice and other information.
"We are, as we always have been, ready to talk at any time to sort out this dispute.
"We have made a very fair and reasonable offer to our staff, but the unions have so far refused to respond or put it to their members.
"The offer remains on the table and we are ready for talks.
"We have today contacted Acas and asked them to assist us in getting back around the table.
"If the unions are serious about resolving this, we trust they will be ready to engage in meaningful talks to deliver night Tube for London.
"We have put forward a very, very fair offer, which consists of an average salary increase of 2%, 1% or RPI (whichever is greater) for next year and the one after, plus a £500 night Tube launch bonus and an additional £2,000 bonus for night Tube train drivers.
"No-one will have to work more hours than they do now, and we have a longer term plan, which will mean no-one will need to work nights if they don't want to."
RMT general secretary Mick Cash said: "The strike action on London Underground is rock solid across all lines and depots and the unity and solidarity of the entire workforce, which has now brought London to a standstill, must force the Tube bosses back to the negotiating table to address the issues at the heart of this dispute.
"That means an end to the attempt to bulldoze through new working patterns that would wreck work/life balance and leave staff in safety critical jobs burnt out and stressed out at a time when Tube services are facing unprecedented demand.
"We've wasted three months in negotiations that failed to address staff concerns and it's essential for London that there's no repeat and that puts the ball firmly in LU's court."
TSSA leader Manuel Cortes called for peace talks to start at Acas tomorrow.
He said: "No-one wants to see London at a standstill, least of all our members, so it is high time for LU to come back to the negotiating table.
"They should stop playing games and start talking to us in good faith to get a sensible solution to this dispute.
"We are ready and willing to be at Acas tomorrow morning to sort this out.
"It is time to end the blame game and agree a solution which keeps London moving and secures the start of the night Tube in September."
London Mayor Boris Johnson cast doubt over the date for the introduction of the night Tube, refusing to rule out that the service would not be launched on September 12, as previously announced by TfL.
"Obviously I very much apologise for all the delay, all the destruction.
"I congratulate the millions of Londoners and others around the City who are making a huge effort to get into work.
"I am very sorry for the Tube being down, we have just got to get through this.
"We have got to get on with the night Tube.
"The unions don't like it, they don't think that we should be able to do this.
"I think it is essential for the city, they want to show that you can't do a huge change like this without them expressing their views.
"Unfortunately what has happened is that the union leadership has basically been spoiling for a big fight on this.
"I think they were very disappointed by the election result in May."
Finn Brennan, of Aslef, said in a message to his members: "For 364 days of the year, London Underground staff work hard to keep this city moving.
"Today you will be vilified by some because you have the courage to stand up for yourselves, the courage to say we will not just allow our employer to impose changes without agreement.
"You will hear time and time again how much benefit the Mayor's plans will bring to London's economy.
"Why is it wrong to say that those benefits should not come at the expense of the people who will work to deliver them?"
Signalling problems at London Bridge railway station compounded the misery for commuters and other travellers.
Mr Cash added: "The Tube management need to take a long hard look at today's shutdown and recognise that they have managed to alienate and anger their entire 20,000 workforce across all grades with the way they have approached the issue of night running.
"Instead of macho, death-or-glory posturing over take-it-or-leave-it offers now is the time for serious and mature negotiations which address the issues at the heart of this dispute. We now expect that to happen as a matter of urgency."