Hundreds Gather at London Vigil for Slain British Politician Jo Cox

— -- Hundreds gathered in Parliament Square in London for a vigil for slain British politician Jo Cox, where speakers urged mourners to "unite against the hate that killed her."

The slaying of Cox has brought an abrupt halt to the campaign for next week's referendum on the U.K.'s membership in the European Union, or the so-called Brexit.

Cox, a British Member of Parliament (MP) and mother of two, was killed in a shooting and stabbing midday Thursday after a town hall meeting in West Yorkshire, England.

Police search teams arrived this morning at the scene in the town of Birstall, where the investigation continues.

Officials confirmed to ABC News today that they were still questioning a 52-year-old suspect and were not looking for anyone else. The police have not released the suspect's name and the man's motives remain unknown.

Cox, 41, was an MP for the Labour Party. She was campaigning for the U.K. to remain in the E.U. and was known for her work on Syria-related issues.

The attack has raised concerns about security measures for British lawmakers. A spokeswoman for the prime minister's office said today that a reminder about safety guidance had been sent out.

Cox reportedly had been subjected to hate mail and police were poised to put extra security in place for her.

Buckingham Palace has said that Queen Elizabeth had written a private letter to the victim's spouse, Brendan Cox, and their two children.

Vigils were held Thursday night as people lit candles and stopped to remember Cox outside Parliament and in her hometown in West Yorkshire.

Flags were flown at half-staff at Houses of Parliament this morning as the Buckingham Palace and other notable landmarks honored the slain politician.

Tributes have also poured in from across the political spectrum.

U.S. presidential candidate Hillary Clinton issued a statement describing the attack as "cruel and terrible"

"It is critical that the United States and Britain, two of the world’s oldest and greatest democracies, stand together against hatred and violence," Clinton said. "This is how we must honor Jo Cox -- by rejecting bigotry in all its forms, and instead embracing, as she always did, everything that binds us together."

Conservative MP Andrew Mitchell wrote an emotional personal tribute in the Telegraph newspaper, describing her as "a five-foot bundle of Yorkshire grit."

"It's hard to believe that someone so brave and fearless and fun is dead, but the hardest thing to think about is her two lovely little children. They would come in to Portcullis House for tea with their mum, and now she’s gone," Mitchell wrote.

Prime Minister David Cameron said in a statement Thursday that the country lost "a great star" who was "compassionate" and had "a big heart."

Campaigning in the referendum on Britain's membership in the European Union, which takes place on June 23, remains suspended.

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