The Transport Enforcement teams were at hubs include Stratford, Canning Town and Victoria.
They were also patrolling buses, trams and DLR trains across the capital.
At Victoria a team of four stopped all those coming through the station maskless and warned them that on the first day a reminder would be given but after that fines would follow.
Officers shouted “face coverings are mandatory on the network” as commuters passed through.
One of the enforcement team said: “Most people listen and are very good and apologise. We have had belligerent people in the past but today we are asking people politely and they are responding well. Maybe they have missed the news or something so we are giving them a chance.”
The team stopped dozens of maskless commuters although the majority of people were wearing masks.
Some were even wearing gas mask-style apparatus as the new Omicron strain of Covid hitting Britain increases concern.
Public address system and poster board warnings emphasised that the non wearing of masks as required would not be tolerated.
Wearing a face mask has been a condition of travel on TfL services since the legal Covid restrictions were relaxed in mid-July but they were not mandatory by law as they had been earlier during the pandemic.
Mayor of London Sadiq Khan has stressed enforcement of face covering rules on the Underground, buses and other services run by Transport for London would be “boosted” after the Government rules kicked in.
Some people are exempt from wearing masks.
Most commuters welcomed the enforcing of the rules.
Margaret Traynor, 60, said: “People should respect the rules. I’m glad TfL are stopping people.”
George Savides, 22, said: “It’s an important message I’m young but of course you have to wear a mask to protect others.”
One man who was stopped told the officers: “I’m sorry I just wasn’t thinking I won’t forget in future.”
Some claimed medical conditions but in general welcomed the intervention of the officers.
At Waterloo notice boards were set up around the station and ticket officers were seen reminding people of the mask rules.
On the Jubilee Underground Line a rough estimate based on observing passengers showed that eight in ten commuters were wearing masks.
London mayor Mr Khan said: “Evidence shows that mask-wearing is the single most effective public health measure at tackling Covid. I welcome the Government’s announcement that face coverings are now compulsory in shops and on public transport nationwide, as they already are across the TfL network. This is a measure I have long been calling for.
“Provided with proper enforcement powers, TfL will now be able to work with policing partners and really boost compliance on the network, with the selfish few who refuse to wear a face covering facing a penalty fine, rather than just being denied entry to transport services. TfL enforcement officers will continue to monitor compliance across the network to keep passengers and staff as safe as possible.”
Siwan Hayward, Director of Compliance, Policing, Operations and Security at TfL, said: “Throughout the pandemic the vast majority of people have been doing the right thing and keeping each other safe from the virus, for which we’re very grateful. The Government has now issued a regulation which means all customers must wear face coverings on public transport unless they are exempt as part of the ongoing fight against Covid.
“Our enforcement officers and our Police partners will enforce this requirement, including denying entry to our services and, with additional powers as a result of the Government’s regulation, issuing a financial penalty to those who refuse to comply. We will continue to monitor compliance across the network and will take action as necessary to ensure that our customers and staff remain safe on our network.”