Britain needs to have "difficult conversations" with Saudi Arabia and other Gulf states about the funding of Islamist extremism, Opposition Leader Jeremy Corbyn says, resuming his election campaign after Saturday's deadly attack in London.

The Labour party leader, who is hoping to win Thursday's national election, said the vote should go ahead to show democracy would not be halted by the London Bridge attack that left seven people dead and at least 48 others injured.

"We must resist Islamophobia and division and turn out on June 8 united in our determination to show our democracy is strong," Mr Corbyn said in a speech delivered in Carlisle, north England.

"And yes, we do need to have some difficult conversations starting with Saudi Arabia and other Gulf states that have funded and fuelled extremist ideology."

The streets of London have seen an increased security presence following the attack. ( Reuters: Peter Nicholls )

Earlier, Prime Minister Theresa May called for a stronger response to Islamist extremism after three knife-wielding assailants drove a hired van into pedestrians on London Bridge and stabbed others nearby.

"Since the emergence of the threat from Islamist-inspired terrorism our country has made significant progress in disrupting plots and protecting the public, but it is time to say enough is enough," Ms May told reporters on Sunday.

"Our society should continue to function in accordance with our values. But when it comes to taking on extremism and terrorism, things need to change."

But during his campaign speech, Mr Corbyn proceeded to attack Ms May for cutting police numbers during her tenure as interior minister and repeated his pledge to recruit 10,000 new police officers, including armed officers.

'Foreign policy connection': political analyst

Opinion polls have shown Mr Corbyn's Labour catching up fast with Ms May's Conservative Party, putting into doubt her gamble that a snap election would boost her majority.

Speaking to the ABC, Inderjeet Parmar, a professor of international politics at the City University of London, doubled down on Mr Corbyn's suggestion that foreign policy was related to repeated terrorism attacks in the West.

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"For the first time, there has been a recognition right across the political spectrum, except within the Government, that there is actually a foreign policy connection in regard to these kinds of attacks that have occurred very recently," Mr Parmar said.

"For example, the Conservative government during the Coalition period had commissioned a report on foreign funding of Jihadi groups and Jihadi terrorism ... that has been put on hold.

"That apparently shows a great deal of sensitive material about some of the powers in the Middle East to which the United Kingdom, and the United States, has been selling arms to for a long time and have been shown to be backing some of the forces which are now behind terror attacks in this country as well."

Last month during a trip to the Middle East, US President Donald Trump urged Arab and Islamic leaders to unite and do their share to defeat Islamist extremists, making an impassioned plea to "drive out" terrorists, while toning down his own harsh rhetoric about Muslims.

Map Where the London incidents occurred

ABC/Reuters