Thousands of protesters descended on Downing Street demanding David Cameron step down in the wake of the row over his personal finances.
Activists waving placards with the slogans "Time to go, chum" and "He's got to go", chanted "Cameron Out" as the Prime Minister delivered his speech at the Conservative spring forum.
The conference in central London took place amid tight security as campaigners also gathered outside the venue, demanding that he quit.
On Whitehall the protesters suspended a pig-shaped pinata, with the prime minister's face stuck on it.
Some turned up wearing pig masks and pig noses, with others sporting tropical shirts and flower garlands in reference to the Panama Papers.
Singer Lily Allen was also spotted amongst the crowd, and she live-tweeted a series of comments and photos from the march using the hashtag #ResignCameron.
One snap showed a campaigner wearing a pig mask and a selfie of the blue-haired singer wearing reflective sunglasses and a leopard-print bandana.
She tweeted on Friday: "I will be protesting at no10 tomorrow, unless of course he resigns before then. You should too, #ResignCameron".
Other banners read "Cameron is a little slug - weasel", "Get the Tories out", and "Eton's Mess".
Calls for Mr Cameron's resignation have been mounting since it revealed he had shares in the offshore trust set up by his late father, which he sold before becoming prime minister.
Following criticism for not making the declaration sooner, he confirmed he would publish his tax returns, accepting blame for the controversy.
Labour leader Jeremy Corbyn has called for the Prime Minister to make a formal Commons statement on the issue, claiming Mr Cameron had "misled the public" and "lost the trust of the British people".