Sir David Amess has been described as “a true gent” and “a common sense politician”.
The 69-year-old, who has been an MP since 1983, died on Friday after being stabbed multiple times at Belfairs Methodist Church in Leigh-on-Sea.
A 25-year-old man has been arrested on suspicion of murder.
Prime Minister Boris Johnson said Sir David was “one of the kindest, nicest, most gentle people in politics”.
Speaking on Friday evening, Mr Johnson said: “I think all our hearts are full of shock and sadness today at the loss of Sir David Amess MP, who was killed in his constituency surgery in a church after almost 40 years of continuous service to the people of Essex and the whole of the United Kingdom.
“And the reason I think people are so shocked and saddened is above all he was one of the kindest, nicest, most gentle people in politics, and he also had an outstanding record of passing laws to help the most vulnerable, whether the people who are suffering from endometriosis, passing laws to end cruelty to animals, or doing a huge amount to reduce the fuel poverty suffered by people up and down the country.
“David was a man who believed passionately in this country and in its future.
“And we’ve lost today a fine public servant and a much-loved friend and colleague, and our thoughts are very much today with his wife, his children, and his family.”
Labour Party leader Sir Keir Starmer said it was a “dark and shocking day”.
“My heart goes out to David’s wife and children, his staff, friends and constituents,” he said.
“This is a dark and shocking day. The whole country will feel it acutely, perhaps the more so because we have, heartbreakingly, been here before.
“Above all else, today I am thinking of David, of the dedicated public servant that he was and of the depth of positive impact he had for the people he represented. Informed by his faith, David had a profound sense of duty, that I witnessed first hand in Parliament. His Catholicism was central to his political life and he was highly respected across Parliament, within the church, and in the Christian community.
“Let us come together in response to these horrendous events. 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.”
Deputy Prime Minister Dominic Raab tweeted: “Heartbroken that we have lost Sir David Amess MP.
“A great common sense politician and a formidable campaigner with a big heart, and tremendous generosity of spirit – including towards those he disagreed with. RIP my friend.”
Chancellor Rishi Sunak tweeted: “The worst aspect of violence is its inhumanity. It steals joy from the world and can take from us that which we love the most.
“Today it took a father, a husband, and a respected colleague. All my thoughts and prayers are with Sir David’s loved ones.”
Home Secretary Priti Patel said she would provide an update “in due course” on questions around the safety of MPs following Sir David’s death.
She said on Twitter: “Questions are rightly being asked about the safety of our country’s elected representatives and I will provide updates in due course. My heartfelt thoughts and prayers are with David’s family and friends.”
Ms Patel added: “I am devastated we have lost Sir David Amess. David was not only a fellow Essex MP, but a kind and loyal friend.
“David served the people of Southend with endless passion, energy and integrity. That he was killed while going about his constituency duties is heartbreaking beyond words. It represents a senseless attack on democracy itself.”
Sir Lindsay Hoyle, Speaker of the House of Commons, said: “I am shocked and deeply distressed by the killing of Sir David Amess.
“David was a lovely man, devoted to his family, to Parliament and his Southend West constituency. He was well-liked by members and the staff alike, and during his almost four decades here, built a reputation for kindness and generosity.
“This is an incident that will send shockwaves across the parliamentary community and the whole country.
“In the coming days we will need to discuss and examine MPs’ security and any measures to be taken, but for now, our thoughts and prayers are with David’s family, friends and colleagues.”
Former Conservative prime minister Theresa May tweeted: “Heartbreaking to hear of the death of Sir David Amess.
“A decent man and respected Parliamentarian, killed in his own community while carrying out his public duties. A tragic day for our democracy. My thoughts and prayers are with David’s family.”
Carrie Johnson, the Prime Minister’s wife, tweeted: “Absolutely devastating news about Sir David Amess.
“He was hugely kind and good. An enormous animal lover and a true gent. This is so completely unjust. Thoughts are with his wife and their children.”
Scottish First Minister Nicola Sturgeon tweeted: “Elected representatives from across the political spectrum will be united in sadness and shock today.
“In a democracy, politicians must be accessible and open to scrutiny, but no-one deserves to have their life taken while working for and representing their constituents.”
On Twitter, Welsh First Minister Mark Drakeford said: “Deeply saddened to hear about the death of Sir David Amess. A truly despicable and horrifying act.
“My thoughts are with his friends, family, and constituency staff during this distressing time.”
Former prime minister Tony Blair said he was “shocked and horrified” by the death and offered his “deepest sympathy and condolences” to his wife and family.
He said: “David and I came into Parliament together in 1983. Though on opposite political sides I always found him a courteous, decent and thoroughly likeable colleague who was respected across the House.
“This is a terrible and sad day for our democracy.”
Former prime minister Sir John Major also paid tribute to Sir David Amess.
In a statement he said: “This is truly heartbreaking news of a good and decent man who – for over 30 years – was a dedicated public servant.
“My heart goes out to his family.”