West Ham overturned a first-half deficit to beat FCSB 3-1 as goals from Jarrod Bowen, Emerson Palmieri and Michail Antonio proved decisive.
The crowd inside the London Stadium paid a moving tribute to Her Majesty Elizabeth II, who passed away on Thursday at the age of 96, during a minute's silence observed before West Ham's first Europa Conference League tie of the new campaign.
West Ham said they shared the "deeply saddened" sentiment of the nation while offering condolences and sympathy to the Royal Family in a social media post prior to kick-off.
Spontaneous choruses of God Save The Queen rang out from the stands, while players from both clubs wore black armbands, as Andrei Cordea opened the scoring in 34th-minute, applying a slick finish to Darius Olaru's low cross at the back post.
Second-half substitute Bowen reduced arrears from the penalty spot in the 65th minute, profiting from Stefan Tarnovanu's clumsy challenge on Maxwel Cornet, before Emerson edged the Hammers in front after pouncing on a goalmouth scramble.
Antonio rounded off the scoring seconds into stoppage time to secure victory for David Moyes' side.
A number of sporting events scheduled for Friday have already been postponed as a mark of respect following Queen Elizabeth II's passing, with further decisions regarding the weekend's fixtures set to be taken on Friday.