It turned red at 6:30pm as part of #RedWednesday, an initiative spearheaded by the advocacy charity Aid to the Church in Need, which is highlighting a lack of religious freedom in countries around the world.
Westminster Abbey, Westminster Cathedral St George's Coptic Orthodox Cathedral (above, with His Grace Bishop Angaelos) are also shining red to mark the occasion.
Other places including town halls, mosques, synagogues are lighting up across the UK in solidarity with persecuted believers as well.
As part of the day a red double-decker bus will visiting iconic landmarks and places of worship across London.
On it will be crossbench peer Lord David Alton and the Patriarch of the Syriac Orthodox Church, His Grace Ignatius Aphrem II, who have both been outspoken about religious freedom. Muslim, Jewish and Sikh leaders were also on the bus calling for the same.
A prayer service with speakers from various faiths was then held outside Westminster Cathedral, the red bus' final stop.
#RedWednesday comes a day before the release of the biennial parliamentary report on persecution, 'Religious Freedom in the World 2016', on Thursday.
Speaking to Premier's Aaron James aboard the Red Wednesday bus, Patriarch Ignatius said: "Obviously it's very emotional for me because it gives me assurance that there are people... of goodwill, of good faith, different backgrounds, coming together to advocate for peace and for the rights of persecuted people - especially calling to bring to justice those that are perpetrating this violence.
"It means a lot to me, it means a lot to the average Syrians who're suffering today under persecution and violence.
"When they know that there are people in London getting together, praying and working for peace in Syria... it means a lot."
And Lord Alton said on the bus: "If you can have campaigns to protect the whale or to safeguard the rights of foxes, then surely we should be doing something about the rights of people who suffer terrible things for their faith.
"If people are dying for their faith then we, who have so many privileges and freedoms and liberties, we should live for our faith. That's what Red Wednesday's all about."
Listen to Premier's Aaron James speaking to Patriarch Ignatius Aphrem II:
Listen to Premier's Aaron James speaking to Lord Alton:
