Mrs Cox's husband Brendan said she would want people "to unite to fight against the hatred that killed her."

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Vote Leave and Remain have both suspended campaigning in the EU referendum in light of the attack.

Mrs Cox, 41, is the first sitting MP to be killed since 1990, when Ian Gow was the last in a string of politicians to die at the hands of Northern Irish terror groups.

The man taken into custody was arrested in Market Street, not far from Birstall Library where Mrs Cox was holding a constituency surgery. He has been named locally as Tommy Mair.

Dozens of Mrs Cox's friends and colleagues gathered later for a vigil at St Peter's Church in Birstall.

Media captionPolice confirmed the death of the MP for Batley and Spen at a news briefingImage copyrightBBC SportImage caption Police said a man was arrested in Market Street near the scene of the attack in Birstall Image copyrightRoss ParryImage caption The arrested man has been named locally as Tommy Mair

Labour leader Mr Corbyn said the country would be "in shock at the horrific murder", describing the MP as a "much loved colleague".

He added: "Jo died doing her public duty at the heart of our democracy, listening to and representing the people she was elected to serve.

"In the coming days there will be questions to answer about how and why she died.

"But for now all our thoughts are with Jo's husband Brendan and their two young children. They will grow up without their mum, but can be immensely proud of what she did, what she achieved and what she stood for."

From the church vigil: Olivia Richwald, BBC Look North

Image copyrightReuters

St. Peter's is a short but wide church and tonight every pew is full.

The vicar says this is a vigil of quietness. A candle with a small flame has been lit in memory of Jo Cox. People of all faiths have their heads bowed. So many are here they're shoulder to shoulder and standing at the back and the sides.

Some stifle silent tears, a lot hold tissues to their faces. The mood is one of shock and almost unbearable sadness.

A Buckingham Palace spokesman said the Queen will write privately to Mr Cox.

Cafe owner Clarke Rothwell, who witnessed the attack, said he heard a "loud popping noise that sounded like a balloon burst - a loud balloon."

Image copyrightReutersImage caption A large police cordon was set up after the attack Image copyrightReutersImage caption Floral tributes left by well-wishers were taken inside the police cordon by officers

"When I looked round there's a man stood there in his 50s with a white baseball cap on and a jacket with a gun, an old fashioned looking gun in his hand," he said.

"He shot this lady once and then he shot her again, he fell to the floor, leant over shot her once more in the face area.