A massive arms fair showcasing technology to tackle everything from militant groups to civil unrest and hackers opened in London on Tuesday, despite attempts by protesters to stop the event.

Held every two years, the four-day Defence and Security Equipment Industry (DSEI) show has grown to five times the size it was when it first opened 15 years ago.

It is a key event in promoting the British arms industry, a plank of the economy that the Conservative government of David Cameron is determined to nurture as it pursues growth.

"It is no exaggeration to say that between September 15 and 18 DSEI will be the point of focus for the global defence and security communities," said DSEI director Duncan Reid.

Britain was the second largest defence exporter in the world in 2013, and has committed to spending at least 2 percent of its gross domestic product on defence, the minimum recommended by NATO.

The fair is split into different areas, with the Security Zone incorporating the highest ever number of first time exhibitors, reflecting the demand for counter-terrorism kit.

Other growth areas include cyber defence, unmanned systems -- including drones -- and military medical innovation, according to organisers.

Peace activists obstructed the set-up of the conference and blocked delegates from entering, and plan protests throughout the week.

Opposition lawmakers challenged Cameron to respond after the Guardian reported that Britain had sold arms to countries that use child soldiers, and that several nations accused of human rights abuses were invited to the fair.

"The UK government has a responsibility to control the arms trade and to work towards stopping the supply to countries using child soldiers or violating any human rights," opposition Scottish National Party MP Brendan O'Hara said in a statement.

- Warships in Thames -

Over the weekend, bus stop advertisements were replaced with posters accusing Britain of selling arms to authoritarian regimes, and adverts on the London Underground were replaced with posters warning commuters about the fair.

"These visitors make huge amounts of money from weapons and equipment that kill people all over the world," the posters read.

"Your taxes help pay for it. Customers are requested to help stop the arms fair."

More than 120 British companies are to be represented at the DSEI, and the conference is to feature a keynote speech by Defence Secretary Michael Fallon.

The ninth edition of the fair, held in the ExCeL exhibition centre, will focus on the security sector and its crossover with defence.

The organisers have welcomed an increase in attendees from the Middle East and Asia, a sign of growing defence budgets in developing nations.

With over 1,500 exhibitors, 42 national pavilions and 50 countries represented, DSEI has established itself as an important event in the defence calendar, alongside France's Eurosatory, organised biannually in Villepinte north of Paris.

While Villepinte largely focuses on ground weapons and equipment, DSEI uses its Thames-side location to accommodate warships.

Eight will feature this year, including from the British, Canadian, German and Indian navies.