The cleaners want a £2 raise to £7.20-an-hour
Cleaners on strike in a row over pay and conditions have been subjected to "appalling" intimidation, a union has claimed.
Hundreds of London Underground cleaners ended their 48-hour industrial action on Thursday evening.
The Rail Maritime and Transport union said it was gathering evidence after allegations that strikers had been bullied and threatened with the sack.
A Transport for London spokesman said it was not aware of any intimidation.
The union's general secretary, Bob Crow, said: "Reports coming in from picket lines over the last 36 hours indicate that the employers are so desperate that they are resorting to gangster-style intimidation and using the worst sort of fear tactics to stop more people joining the strike.
"Managers have been threatening people with the sack if they join the strike and telling them that they will have sums deducted from their wages that are way above what they could have earned during the strike."
But a Transport for London (TfL) spokesman said: "We are not aware of any instances of intimidation having taken place.
"Indeed, there has been very limited picketing around the network during the strike and the majority of cleaning staff have gone about their jobs as normal with no disruption to our services."
Meanwhile, feminist activists staged a protest outside a Tube station near Westminster in support of the cleaners, most of whom are women and people from ethnic minorities.
The women said they dumped rubbish outside St James's Park during the protest.
TfL said police were called to the incident, adding that it had not disrupted services.
The union is seeking a so-called London Living Wage of £7.20 an hour, almost £2 more than some cleaners said they earn, as well as improvements to holidays, sick pay and pensions.