Image source, AFP

Liberal Democrat leader Tim Farron has responded to Theresa May's campaign speech in Leeds earlier, accusing her of having "betrayed the national interest by opting for a disastrous hard Brexit that will wreck our economy". 

He adds: "Now she is posturing to try and win votes at home, instead of building bridges abroad with our allies in Europe. 

"It is shameless opportunism that has nothing to do with May's hand in the Brexit talks and everything to with attempting a naked power grab."

The Lib Dems are campaigning against a "hard Brexit" that would take the UK out of the single market and end free movement of people - and for a second referendum on the terms of any Brexit deal reached with the EU.  

Here's a quick look back at the stories of the day: 

 A man carrying knives near the Houses of Parliament is wrestled to the ground by armed police and arrested on suspicion of terrorism offences.  

Prime Minister Theresa May tells party supporters in Leeds that comments by the German chancellor about some Britons' "illusions" about Brexit show the need to strengthen her hand with a strong mandate.

Boris Johnson says it would be very difficult for the UK to refuse the US if it asked for support in another military strike on Syria.

The foreign secretary also launches a personal attack on Labour leader Jeremy Corbyn - calling him a "mutton-headed, old mugwump".

Labour says it will build one million homes in five years, half of which would be social housing, if the party gets into power.

The UK Government loses a court bid to delay publication of its air pollution strategy, and must now release it before the June election.   

Questioned by journalists at the end of her speech, Mrs May was challenged about whether she agreed with her foreign secretary that the UK could join the US in another military strike on Syria, possibly with a vote in Parliament.

Mrs May said it was a "hypothetical issue" as there were no proposals on the table for a further strike on Syria.

She said it was important to work towards a political transition away from Syrian President Bashar al-Assad.

She laughed off a question about whether Boris Johnson would remain foreign secretary if she wins the general election, adding: "Boris is doing a great job as foreign secretary."

Mrs May goes on to say that the election is important to provide a government "that can take us not just through Brexit, but beyond Brexit". 

And she points out to the crowd in Leeds that polls were wrong about the 2015 election, the EU referendum and mentions how Jeremy Corbyn was a 200-1 candidate in the Labour leadership battle.

"So every single vote counts, we must not be complacent," she says. 

Mrs May also says the election is not about "who you might have voted for in the past", but voting "in the national interest".

She goes on to say that, ahead of the Brexit negotiations with the EU, the nation needed "the strongest possible hand, the strongest possible mandate and the strongest possible leadership".

Theresa May, speaking at a campaign rally in Leeds, says the general election is "the most important election this country has faced in my lifetime.

She added: "And in this election, every single vote will count. And every person in this country has a positive reason to lend me their vote."

We're leaving the Whitehall incident for now, but will keep you posted if there are any new developments.

Meanwhile, back on the general election trail, there's a few things to update you on.

• UKIP leader Paul Nuttall has confirmed he will stand for Parliament in the general election, but has not revealed which seat he will contest 
• Rachel Johnson, the foreign secretary's sister, has become a member of the Lib Dems, according to Channel 4 News, external. 
• Theresa May has responded to Angela Merkel's comments about the UK's Brexit stance. The German chancellor has said some Britons had "illusions" over trade talks running alongside negotiating the terms UK's exit. The PM now claims Germany's position proved she needed "the strongest possible hand" in the talks
• The House of Commons and House of Lords are now officially prorogued and will not meet again until the State Opening of Parliament

Image source, AFP

Another eyewitnesses, Chris Kacyk, told the BBC he saw the incident on Whitehall and that "one of the objects looked like a knife". 

He added: "It looked like a knife with about a thirteen centimetres long blade. There were other... knife-looking objects. I would say about five knives I could see."

Radio 4 PM

Image source, PA

BBC correspondent Andy Moore tells BBC Radio 4's PM programme the arrested man is 27-years-old and is believed to live in London.

According to sources he is a British passport holder but not believed to be born in UK, Andy said.

Image source, Reuters

Ian Moss was travelling on a bus through Whitehall at the time of the incident. He says he saw the suspect on the ground in the traffic island in Whitehall. 

Mr Moss also said police "looked like they were tending" to him. 

"He was arrested. He could walk so no idea if he was injured." 

Image source, Getty Images

A 27-year-old man has been arrested in Westminster following a stop and search operation by police. The man is being held under the Terrorism Act. Police say knives were recovered from him following the operation on Thursday afternoon.  

BBC News Channel

The prime minister said she was "aware that an individual has been arrested" and that he was detained "on the basis of a terrorism charge". 

She said she could "not say much more about it because it is an ongoing police investigation".

However, she added: "It shows that our police and our intelligence and security services are on the alert as they always are" and that "we owe a huge debt of gratitude to these people".

Image source, AFP/Getty

A number of items were left on the ground following the incident in Whitehall.

Following a painstaking routine, two police forensics officers, covered entirely in crime scene suits apart from their eyes, slowly photographed and picked each one up.

Several knives and what looked like paper or tissue were all put into a separate bag or box. When it came to the brown rucksack lying on the floor, the officer opened it up somewhat gingerly.

Their every move was followed by snicker of camera lenses, both from the press and tourists who were all craning their necks to see what would happen next.

One of the tourists, nodding towards the discarded objects, asked if someone had died.

And above it all was the incessant clucking of a police helicopter, making everyone raise their voice, the tension biting as hard as the cold winds.

The man detained in London was known to counter-terrorism police and British intelligence before his arrest, Reuters says.

He was arrested on suspicion of possession of an offensive weapon and on suspicion of the commission, preparation and instigation of acts of terrorism. He is being questioned at a South London police station.

Theresa May was not in Downing Street during the police operation in Whitehall, her spokesman said, but she was informed about it by officials.

A French tourist, who was in New York on 9/11 and was visiting London with her daughter, saw the arrest happen. 

The witness, who declined to be named, said: 

The tourist added: "I was in New York on September 11, so I am a bit scared of these things and being in the wrong place at the wrong moment." 

BBC News Channel

Image source, EPA

Speaking from the scene in Whitehall, central London, the BBC's Andy Moore said the police had told him there was no ongoing terror threat.

In the last half-hour boiler-suited forensic officers had carried out a "delicate evidence gathering operation", he said, picking over items on the ground, including two knives - one large bread knife - and a brown backpack.

The operation started around 14:20 BST with armed police officers stopping a man in his 20s on a traffic island just outside Westminster tube station.

"We don't know why he was stopped, but he was known to the police," Andy said, adding that the individual was hand-cuffed with one of his hands bandaged.

Image source, Reuters

A photo shows a man with what appears to be a bandage around his right hand in handcuffs with two police officers after an arrest was made in Whitehall.