LU managing director Mike Brown said:
"Many thousands of LU and TfL staff are working hard again today to keep customers informed and to keep London moving and open for business.
"I am grateful to our customers for their patience, and we will be doing everything we can to run as many services as possible. I'm pleased that more staff have come to work today than yesterday.
"My message to the leadership of the RMT and TSSA is 'why wait to sit down with us until Friday?'. They should do the right thing, meet us at Acas today, and suspend this completely unnecessary strike.
Transport for London are operating a limited service on most tube lines, but they advise to check before you travel.
Union leaders were under pressure tonight to call off the strike after it emerged new peace talks would take place on Friday.
Bob Crow has told ITV News that he does not use ticket offices at London Underground stations.