Image caption The Islington pub where Bolsheviks and Mensheviks argued fiercely during the 1903 party congress

In August 1903, a small band of dedicated but argumentative political activists held a fractious conference in London.

It consisted of Vladimir Lenin, Leon Trotsky and about 50 other committed agitators who wanted to overthrow the autocratic rule of the Russian Tsar. Their quarrels might have seemed minor at the time, but they have rippled out across history.

This was when the Russian revolutionary movement divided into the two rival factions of Bolsheviks and Mensheviks. And a key vote happened in a pub in Islington.

The Bolsheviks, described as the 'hards' and led by Lenin, wanted a tightly centralised and disciplined political party; the Mensheviks or 'softs' favoured a looser, broader-based alliance with sympathetic forces. Over the following years, as issues and affiliations shifted, their differences fluctuated but were to become deeper.

Fourteen years later, in the second (October) revolution of 1917, the Bolsheviks took power, sidelining and defeating the Mensheviks, and went on to form the Soviet Union.

At the 1903 congress in London, where the split emerged, Lenin's faction narrowly lost the vote on the nature of party membership.

But then seven anti-Lenin delegates walked out over other disagreements, and with his opponents depleted, his side then won a crucial vote on the editorial board for the party's journal.

Pub venues

This outcome enabled Lenin to call his group the Bolsheviks, meaning 'majority' in Russian, while his rivals became the Mensheviks or 'minority'.

The bitter dispute prompted uproar in the meeting. According to Richard Mullin, a researcher into early Russian Marxism, Lenin's notes indicate that the tumultuous session took place in the Three Johns pub in Islington.

Image caption The Whitechapel building where Lenin, Stalin, Trotsky and others arrived for the 1907 congress

"The 1903 London congress is regarded as decisive in the development of Bolshevism - it's hugely significant", says Neil Faulkner, author of A People's History of the Russian Revolution.

But of course its significance is seen differently according to different political viewpoints.

"Most people on the revolutionary left would say this is the decisive break between revolution and reform," explains Dr Faulkner.

"A lot of liberal commentators would see it as the tiny seed from which ultimately grows the gulags and the labour camps of the 1930s."

To avoid being monitored during their conference, the Russians moved from venue to venue over a fortnight, often using meeting rooms in pubs recommended by friendly British trade unionists.

Image copyright Getty Images Image caption Leon Trotsky met Lenin for the first time in London

The first session in London occurred in a club in Charlotte Street in central London. Otherwise most of these locations are unknown today.

The 1903 congress had actually started in Brussels, but after harassment from the Belgian police it moved to London. The British authorities showed more acceptance of exiled Russian revolutionary activities than did many other European countries.

This comparative tolerance meant that some other key events in the history of the Russian revolutionary movement also happened in Britain.

The 1907 party congress moved to London after being banned in Denmark, Sweden and Norway. This was a much bigger affair of more than 300 delegates, following an outbreak of major social unrest against the Tsar in Russia in 1905.

The congress took place in the Brotherhood Church in Hackney, which has since been knocked down and replaced by a housing development.

Those present included almost all the future leaders of the Bolshevik revolution, including Lenin, Trotsky, Stalin (a minor figure at the time), Zinoviev, Kamenev and Litvinov, as well as the prominent Russian writer Maxim Gorky. This was the last full congress of the party until after the revolution.

Image caption Lenin briefly lived in Tavistock Place in Bloomsbury, central London in 1908

The participants first registered for the conference at a building in Fulbourne Street, Whitechapel, which still stands today. At the time it was a Jewish socialist club.

Stalin and Maxim Litvinov (who later became Soviet foreign minister) stayed in a doss house nearby in Fieldgate Street, which has since been converted into a somewhat more salubrious block of flats.

The conference saw further disputes between Bolsheviks and Mensheviks. One issue for discussion was whether to approve the use of bank robberies to help fund revolutionary activities.

Most delegates could only afford the trip back to Russia when the impoverished party secured a loan from an eccentric soap manufacturing London businessman who was inspired by watching conference proceedings.

A few years earlier Lenin had spent 12 months in London, in 1902-3. He mainly divided his time between researching and writing at the British Museum reading room, and editing a revolutionary journal, Iskra ("The Spark").

'Lenin room'

In the reading room he studied works on economics and on the Russian peasantry.

Lenin was able to obtain books which would have been confiscated in Russia, and was rather impressed by the British state's commitment to the library, telling a friend: "The British bourgeoisie do not spare any money as far as this institution is concerned, and that is as it should be."

On his various visits to London, Lenin generally stayed around the Bloomsbury area, so that he had easy access to the museum.

In 1902 Iskra was produced in London and smuggled across Europe into Russia. Lenin was provided with an office and printing facilities by a supportive left-wing publishing company.

Image caption The 'Lenin room' in the Marx Memorial Library

This building is now the Marx Memorial Library in Clerkenwell. They have preserved what they call 'the Lenin room' with busts of him, old editions of the journal, and copies of Lenin's voluminous collected works.

A map on the wall outside shows the smuggling routes used. For Lenin, the journal was crucial both for building up a network of revolutionary activists and also for spreading the political analysis he favoured.

It was in London, in October 1902, that Lenin and Trotsky met for the first time. The pair discussed the political circumstances of Russia, but Lenin also showed Trotsky the sights of London.

When they went past the Houses of Parliament, Lenin said to his companion "that is their famous Westminster".

Trotsky later wrote it was obvious that by "their", Lenin didn't mean it was the British parliament, he meant it was the ruling class's parliament.

Yet it was that parliament, and the system it represented, which gave Lenin, Trotsky and their comrades the political freedom to pursue their goals.

Martin Rosenbaum is the presenter of The British Road to Bolshevism on BBC Radio 4 at 20:00 on Monday, 16 October 2017. You can follow Martin Rosenbaum on Twitter as @rosenbaum6