So-called "420 Day" came to London's Hyde Park at the weekend.

Red police signs saying "Cannabis Is Illegal" greeted thousands of pro-cannabis supporters arriving for the rally.

They were there to hear speeches on decriminalisation, but also to smoke cannabis. The organisers wanted a "rethink of how the drug is treated in Britain".

More than 50 people were arrested by police, 37 for drugs-related offences.

"420" day is held on the 20th April, but is actually named after 4:20pm.

In 1971 a group of friends in California, trying to find a mystery marijuana crop marked on a map they found, began meeting up after school at 4:20pm to smoke cannabis.

They called themselves the Waldos (because they smoke by a wall).

420 became code for cannabis.

Some of the clocks in the film Pulp Fiction are set to 4:20pm and a sign in Colorado in the US had to be changed to 419.99 miles to stop people stealing it.

"We prefer to look at this as a health issue rather than a criminal issue", says the 420 event's organiser Stewart Harper.

It is a criminal offence to smoke cannabis . Possession of Class B drugs has a maximum sentence of 5 years in prison.

Who says what on drugs

Conservatives

David Cameron said he does "not believe in decriminalising drugs that are illegal today.... I don't want to send out a message that somehow taking these drugs is OK or safe."

He said that under his government drugs use is falling because "we've been focusing on education, prevention and treatment, and that is the right approach to take".

Labour

"If the government sends a signal that somehow cannabis is okay - and we know there are hard forms of cannabis and softer forms - I think it's sending the wrong signal", says the Labour leader Ed Miliband.

There are "mental effects of cannabis that people maybe didn't realise a decade ago".

Liberal Democrats

In their manifesto they say they want to put people who are arrested with drugs for personal use into "treatment, education or civil penalties that do not attract a criminal record".

They still want "severe penalties" for those who make or deal illegal drugs, and want to "review" the effectiveness of the cannabis legalisation in parts of the United States and Uruguay.

UKIP

"We will not decriminalise illegal drugs, however we will focus on ensuring drug suppliers, not their victims, face the full force of the law."

Green Party

"Treat drug addiction as a health problem rather than a crime." They want to look again at how drugs are classified and bring the market under government control.

Follow @BBCNewsbeat on Twitter, BBCNewsbeat on Instagram, Radio1Newsbeat on YouTube and you can now follow BBC_Newsbeat on Snapchat