MPs and peers have paid tribute to Labour MP Jo Cox, who was killed in her constituency in West Yorkshire on Thursday.
Parliament, which was in recess for the EU referendum, was recalled to remember Mrs Cox, 41, who was described as "perfect" by her family.
Politicians are now at a memorial service at St Margaret's Church.
Speaker John Bercow said the Commons had gathered in "heartbreaking sadness" but also in "heartfelt solidarity".
A charity fundraising appeal set up in Mrs Cox's memory has now raised more than £1 million.
As he led the tributes in a packed House of Commons, Mr Bercow said Mrs Cox had "outstanding qualities... she was caring, eloquent, principled and wise".
"Above all she was filled with and fuelled by love for humanity. Devoted to her family and a relentless campaigner for equality, human rights and social justice," he added.
Mr Bercow added: "An attack like this strikes not only at an individual, but at our freedom. That is why we assemble here, both to honour Jo and to redouble our dedication to democracy."
Mrs Cox's husband, Brendan, and their two children were in the public gallery to hear MPs' tributes, along with Mrs Cox's parents Jean and Gordon, sister Kim and other family members.
A white and a red rose were placed in Mrs Cox's usual place on the Labour benches.
Speaking next, Labour leader Jeremy Corbyn told MPs, who were wearing white roses in memory of Mrs Cox: "We have lost one of our own and society as a whole has lost one of our very best."
He said Mrs Cox - whose 42nd birthday would have been on Wednesday - had spent her life serving and campaigning "for other people" - at home and abroad - both in her charity work and as an MP.
He called her death "an attack on democracy and our whole country has been shocked and saddened by it... and was united in grief".
He appealed for "a kinder and gentler politics", saying politicians had a responsibility "not to whip up hatred or sow division".
In her honour, he said "we can come together to change our politics to tolerate a little more and condemn a little less".
Prime Minister David Cameron described Mrs Cox, the MP for Batley and Spen, as an "extraordinary colleague and friend", who was a humanitarian "to her core" who brought out "the best" in people.
"She was a voice of compassion, whose boundless energy lit up the lives of all who knew her and saved the lives of many she never met."
He said her killing was a "sickening and despicable" act and, like the speakers before him, offered condolences to her family.
Mr Cameron added: "We pay tribute to a loving, determined, passionate and progressive politician, who epitomised the best of humanity and proved so often the power of politics to make the world a better place."
Several MPs could be seen in tears as tributes were made from across the House to Mrs Cox as a charity worker, an MP, a wife and a mother.
She was "struck down too soon", said Labour MP for Leeds West Rachel Reeves, who told MPs "it now falls on all of our shoulders to carry on Jo's work - to combat and guard against hatred, intolerance and injustice and serve others with dignity and love".
Ms Reeves added, to tears, that Batley and Spen would go on to elect a new MP, "but no one can replace a mother".
Conservative MP Andrew Mitchell said Mrs Cox was a "truly exceptional woman" whose "goodness and passionate dedication to humanitarian values has inspired us all."
He said she would not want her death to change the "open and accessible relationship" MPs enjoy with their constituents, but she would want the UK to redouble its efforts to resolve the Syrian crisis, which he called "the greatest catastrophe of our age".
For the Lib Dems, leader Tim Farron said MPs stood "united" to mourn the death of "an enormous figure in this House".
He added: "The snatching away of a wife and a mother, hugely loving and hugely loved, is what has moved Britain to stand in collective grief this last few days."
"A proud Yorkshire lass," was how Conservative MP Stuart Andrew remembered Mrs Cox. He said he would miss her "passion and conviction" but above all "her smile".
Meanwhile, Stephen Kinnock, who shared an office with Mrs Cox, a friend for 20 years, said Mrs Cox gave "a voice to the voiceless and spoke truth to power" and exemplified the "best values" of Labour and the country.
Echoing the sentiment of others, he said her legacy must be a politics of "hope note fear, respect not hate, unity not division".
In very rare and emotional scenes in the Commons chamber, MPs broke in to applause in memory of the MP as they filed out of the chamber, many comforting each other as they wiped away tears.
The MPs then proceeded to St Margaret's Church, alongside members of the House of Lords, to St Margaret's Church, where a short memorial service will take place for Mrs Cox.
A church service was held in Batley and Spen on Sunday to remember the life of Mrs Cox, who was described as a "21st Century Good Samaritan".
Mrs Cox's sister Kim Leadbeater has thanked the community for their support, saying it had "genuinely made a difference" and helped the family through some "dark times".
Friends of Mrs Cox have announced plans for a public event in Trafalgar Square in London on Wednesday.
There have also been calls for a memorial to be installed at Westminster to remember her.
Mrs Cox is the first sitting MP to be killed since Conservative Ian Gow was blown up by the IRA in 1990.
Thomas Mair, 52, from Birstall, faces charges of murder, grievous bodily harm, possession of a firearm with intent to commit an indictable offence and possession of an offensive weapon in connection with the attack on Mrs Cox.
He refused to give his correct name and did not reply when asked to confirm his address and date of birth at an appearance at Westminster Magistrates' Court on Saturday.
When asked to confirm he was Thomas Mair at the Old Bailey on Monday, he replied: "Yes, I am."
There was no application for bail and he was remanded in custody. He will appear at the same court for a preliminary hearing before Mr Justice Saunders at 10:00 BST on Thursday.