Counting is continuing in the Tooting by-election to decide the Commons' newest member, but the murder of Labour MP Jo Cox hung heavily over proceedings.
A two-minute silence was observed at the count as the death of the Batley and Spen MP cast a shadow over the process.
Labour councillor and junior doctor Rosena Allin-Khan is seeking to retain the seat for Labour after it was vacated by Sadiq Khan following his election as London's mayor.
Her main rival for the south London seat is Tory Dan Watkins, who unsuccessfully stood against Mr Khan in the general election last year.
Mayor of Wandsworth Richard Field paused the count to observe the silence at 11pm, an hour after the polls closed.
For two minutes the excited chatter and rustling of ballot papers stopped as people bowed their heads.
Taking to the stage at Wandsworth Town Hall, Mr Field said: "The impact of this tragic event is being felt right across the country, but it has a very deep resonance here in this room as the votes are counted for Tooting's new member of parliament."
Turnout in the by-election was 42.5%, Wandsworth Council said.
Mr Khan stood down as MP for Tooting after defeating Tory Zac Goldsmith in the City Hall contest in May.
Dr Allin-Khan and Mr Watkins have both described Mrs Cox's death as "shocking" and said their thoughts and prayers were with her family and friends.
Mr Khan, who had been the area's MP since 2005, held Tooting with a majority of 2,842 over Mr Watkins - a local businessman - at the general election.
The by-election result will be closely watched by Jeremy Corbyn's critics in the Labour Party, but coming a week before the EU referendum, the campaign has been overshadowed by the closing stages of the Brexit battle.
Born and raised in the area, Dr Allin-Khan works as an emergency doctor at St George's Hospital and is also deputy leader of Wandsworth Labour group.