A group of demonstrators also targeted the headquarters for Shell on Monday, spraying phrases such as "Shell knows" and "climate criminals" onto the front of the building.
"Met Police say three men have been arrested so far on suspicion of criminal damage".
The demonstrations are part of a global campaign, organised by the British climate group Extinction Rebellion, which encompasses protests in 80 cities, across 33 countries, in the coming days.
Trees and trucks were used to block the busy Thames crossing as the protests kicked off.
Protesters parked a pink boat in the middle of Oxford Street - one of Europe's busiest streets - as a flag bearing the symbol of the organisers, Extinction Rebellion, was raised.
Others were present in Parliament Square where they unfurled banners, held up placards and waved flags as speakers gave speeches on stage.
Environmental consultant Dr Andrew Boswell, former Norfolk county councillor, joined activists at Marble Arch and said any inconvenience people experienced as a result of the two week protest was insignificant in comparison to the "catastrophic impact" of climate change.
Yen Chit Chong, from Extinction Rebellion in London, said: "This is our last best shot at survival".
It is calling on the government to reduce carbon emissions to zero by 2025 and establish a citizens' assembly to devise an emergency plan of action similar to that seen during the second world war.
"We are still in this position, and still facing this crisis", she said.
Activists gathered at Hyde Park yesterday and were encouraged to camp there overnight before a march towards Piccadilly Circus today.
"They can change a few of the laws along with the Paris agreement so that we can really work towards achieving a practical target".
Extinction Rebellion said protests would continue throughout the week "escalating the creative disruption across the capital day by day".