Two roses, one red and one white, have been placed in the House of Commons seat of the Labour MP Jo Cox.
The House of Commons met in "heartbreaking sadness" and "heartfelt solidarity" to pay tribute to Jo Cox, Speaker John Bercow said.
Mr Bercow said the killing of the Labour MP - "in this manner, of this person, our democratically elected colleague" - was "particularly shocking and repugnant".
MPs of all sides wore a single white rose as a mark of respect.
Liberal Democrat leader Tim Farron said "getting this Lancastrian to wear this white Yorkshire rose" was an achievement in itself.
Several of Ms Cox's Labour colleagues, including Heidi Alexander, Stephen Kinnock and Carolyn Harris, were in tears before Mr Bercow began the tributes.
MPs were allowed to break with tradition to sit together, rather than being divided along party lines, in a gesture of solidarity proposed by Tory MP Jason McCartney.
A petition to cancel next week's European Referendum has seen a surge in support in the days following her killing. Over 53,000 people have signed the statement on the Parliament website.
A GoFundMe page set up raise money for charities Ms Cox supported has received over £933,000 in three days.
Following the tributes, Labour and Conservative MPs will walk together in pairs to St Margaret's Church - often referred to as the "parish church of the Commons" - for a service of prayer and remembrance.
Additional reporting by Press Association
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