Prime Minister Malcolm Turnbull has condemned the deadly terrorist attack outside London's Houses of Parliament, declaring it an "attack on parliaments, freedom and democracy everywhere".

Key points: Attack in central London left five dead, 40 injured

Attack in central London left five dead, 40 injured Malcolm Turnbull says the attack is "an assault on every democracy, every parliament, every free nation"

Malcolm Turnbull says the attack is "an assault on every democracy, every parliament, every free nation" Parliament House in Canberra is increasing security measures in response to the incident

Extra police will patrol Parliament House in Canberra today after the attack, which left at least five people including the attacker dead, and another 40 injured.

The suspected terrorist used a car to mow down pedestrians on Westminster Bridge before fatally stabbing a police officer outside the Houses of Parliament.

"Almost every element of our parliamentary tradition here in Australia is modelled on that of the Houses of Parliament in London — the birthplace of our great, free, parliamentary democracy," Mr Turnbull said.

"This is an assault on every democracy, every parliament, every free nation."

Security chiefs including Australian Federal Police Commissioner Andrew Colvin have briefed Prime Minister Malcolm Turnbull.

"The AFP Commissioner has confirmed there will be an increase in the police present at Parliament House here in Canberra today," Mr Turnbull said.

"That is an appropriate response.

"It indicates the care and the caution that is taken by our security agencies in keeping us safe.

"We will continue to monitor the situation very closely."

Opposition Leader Bill Shorten said his thoughts were with "Australia's oldest friend, the United Kingdom".

"Our sympathy goes out to the victims, in particular the policeman who died in the line of duty and those others who were slain.

"The terrorists should know, whenever they are, they will never divide this country."

Talks with spy agency boss, Chinese leader visiting

Mr Turnbull has also spoken with head of the Australian Security Intelligence Organisation, Duncan Lewis, and Australia's High Commissioner to the UK, Alexander Downer.

Chinese Premier Li Keqiang is visiting Parliament today as he begins a five-day visit to Australia.

Heavily armed AFP officers already guard Parliament House 24 hours a day.

A fence is also being constructed on Capital Hill to restrict public access to Parliament's roof.

Australia's terrorism threat remains at "probable".