The killing of Conservative British MP Sir David Amess has been declared a terrorist incident by police, as Prime Minister Jacinda Ardern expressed New Zealand's condolences.
Sir David was stabbed multiple times at a constituency surgery in Leigh-on-Sea in Essex on Friday.
The Metropolitan Police said there was "a potential motivation linked to Islamist extremism".
A 25-year-old British man was arrested at the scene on suspicion of murder, and police said they were not seeking anyone else over the death.
In a statement today, Jacinda Ardern called the murder "devastating."
"I send New Zealand's thoughts to the family, friends and constituents of Sir David Amess MP. No matter where you are in the world, those who represent their communities in parliament should be safe.
"To hear that someone has lost their life while serving their people is devastating. We send both our love to Sir David's family, and our condemnation for this act of violence."
National Party leader Judith Collins also expressed sadness, calling the killing shocking and deeply disturbing.
She said it is especially tragic that, in performing his duty to the best of his ability, David Amess has paid the ultimate price. Collins said National condemns the senseless act of violence.
As part of the investigation, officers are currently carrying out searches at two addresses in the London area, the Met said.
The force believes the man acted alone but inquiries into the circumstances of the incident are continuing.
Government sources have told the BBC the man is a British national who, from initial inquiries, appears to be of Somali heritage.
Prime Minister Boris Johnson said "our hearts are full of shock and sadness" at the loss of "one of the kindest" people in politics.
Sir David, 69, had been an MP since 1983 and was married with five children.
The investigation will be led by the Metropolitan Police's specialist Counter Terrorism Command, who will determine whether it was a terrorist incident.
Johnson said Sir David had an "outstanding record of passing laws to help the most vulnerable".
"David was a man who believed passionately in this country and in its future. We've lost today a fine public servant and a much-loved friend and colleague," he said.
Health Secretary Sajid Javid said he was "a great man, a great friend, and a great MP, killed while fulfilling his democratic role".
Home Secretary Priti Patel said the killing "represents a senseless attack on democracy itself", adding that "questions are rightly being asked about the safety of our country's elected representatives".
Sir David, who represented Southend West, was holding a constituency surgery - where voters can meet their MP and discuss concerns - at Belfairs Methodist Church in Eastwood Road North.
Essex Police Chief Constable Ben-Julian Harrington said they received reports of a stabbing shortly after 12:05 BST and within minutes, officers found Sir David with multiple injuries.
Police and paramedics "worked extremely hard" to save him but he died at the scene, Harrington said.
Police have appealed for anyone who saw the attack, or who has footage from CCTV, dash cams or video doorbell to contact them.
Sir David is the second MP to be killed in the past five years, following the murder of Labour MP Jo Cox in 2016.
She was killed outside a library in Birstall, West Yorkshire, where she was due to hold a constituency surgery.
Southend councillor John Lamb told the BBC that Sir David moved his surgeries to different locations around the constituency "to meet the people" and said the attack was "absolutely dreadful".
"We've lost a very good, hard-working constituency MP who worked for everyone," he said.
Father Jeff Woolnough, parish priest at nearby St Peter's Catholic Church, told the BBC: "Sir David was a great, great man, a good Catholic and a friend to all...
"He's died doing that, that's the remarkable thing. He's died serving the people."
Another priest, Father Kevin Hale, said there was a "great sense of incredulity" about the attack.
Speaker of the House of Commons Sir Lindsay Hoyle said: "This is an incident that will send shockwaves across the parliamentary community and the whole country."
He said he was shocked and deeply distressed by the killing of a "lovely man", and said in the coming days they would need to examine MPs' security.
Labour leader Sir Keir Starmer said it was a "dark and shocking day", adding that "we have, heartbreakingly, been here before" with the death of Jo Cox.
"We will show once more that violence, intimidation and threats to our democracy will never prevail over the tireless commitment of public servants simply doing their jobs," he said.
Sir David wrote about the impact on MPs of Jo Cox's murder and a machete attack on Nigel Jones MP in 2000, which led to the death of his aide Andy Pennington, in his diaries published in November.
Referring to that machete attack, he said: "We all make ourselves readily available to our constituents and are often dealing with members of the public who have mental health problems, it could happen to any of us."
Liberal Democrat leader Sir Ed Davey said it was "a truly terrible day for British politics but most importantly of all our prayers are with all the people who loved David".
Flags around Parliament and at 10 Downing Street are being flown at half-mast.
Archbishop of Canterbury Justin Welby said the MP's death was "agonisingly painful" for those who knew him.
"The murder of an MP, in the course of caring for their constituents, is a deep blow to this country, its citizens and everyone who desires a peaceful and flourishing democracy," he said.
The Archbishop of Westminster Vincent Nichols, the leader of the Roman Catholic Church in England and Wales, said Sir David "carried out his vocation as a Catholic in public life with generosity and integrity".
"This horrific attack, as David was undertaking his constituency surgery, is an attack on our democratic process and traditions," he said.
Flowers were left near the scene of the killing, with tributes describing Sir David as "kind and thoughtful" and "such a gentleman".
What security do MPs have?
- When they are in Parliament, MPs are well protected by a specialist armed police department called Parliamentary and Diplomatic Protection
- For most MPs there is not the same close protection when they are in their constituencies
- Parliament offers MPs and their staff guidance on security, including tips on how to run a safe constituency surgery, suspicious post and home security
- Parliament will pay for MPs to have some security measures installed - such as security alarms and shutters
- After the murder of Jo Cox in 2016, the spending on MPs' security rose from £170,576 in 2015/16, to £4.5m two years later
- BBC w/ RNZ