Labour's Rosena Allin-Khan has won the Tooting by-election in a contest overshadowed by the murder of MP Jo Cox.
Dr Allin-Khan held the south London seat for Labour after London mayor Sadiq Khan stood down following his victory in the City Hall race in May.
The A&E doctor, who increased the Labour majority, used her acceptance speech to pay tribute to Mrs Cox, who was killed in her Batley and Spen constituency on Thursday.
She said Mrs Cox was a "proud and passionate campaigner who will be desperately missed".
A two-minute silence was held during the count and the flag was flown at half-mast over Wandsworth Town Hall in memory of Mrs Cox.
Dr Allin-Khan increased Labour's majority to 6,357 over the Conservatives, up from the 2,842 secured by her predecessor in the general election last year.
In a sombre statement following her victory at Wandsworth Town Hall she referred to the "horrific events of yesterday and the shocking death of Jo Cox".
She added: "My thoughts and prayers are with Jo's husband and her children. She was a proud and passionate campaigner who will be desperately missed.
"Jo's death reminds us that our democracy is precious but fragile. We must never forget to cherish it.
"Thousands of people voted today and we are all here in recognition of our democratic values."
Before the contest, 39-year-old Dr Allin-Khan promised to put the NHS and housing at the centre of her agenda if she won.
Tooting has been regarded as a Labour stronghold since 1974.
Last year, Mr Khan retained the seat with 25,263 votes.
