"Tube: severe disruption on all lines. Little to no services throughout the day. No Night Tube," the operator said.
The strikes have also affected surface rail, trams and buses, the statement said, adding that some services have been suspended or reduced.
The United Kingdom has been facing a wave of strikes in recent months due to record inflation in the country. Annual inflation rose to a new 40-year high of 10,1% in July from 9.4% in June, according to the UK Office for National Statistics.
On Saturday, several UK trade unions will again go on strike, including the National Union of Rail, Maritime and Transport (RMT). Also on that day, more than 1,600 bus drivers in London plan to go on strike. On Sunday, an eight-day strike will begin in the UK's largest container shipping port, Felixstowe.