Activists from the Extinction Rebellion group unfurled banners with slogans including "Climate Criminals" and dumped the blackened boulders in the road outside the building on Lime Street in the heart of the City of London.
The group said that Lloyd's facilitated the fossil fuel industry.
"The dumped coal highlighted Lloyd's support for the most polluting projects – tar sands and coal mines," notably Adani Enterprises' giant Carmichael thermal coal mine in Australia, Extinction Rebellion said in a statement.
Lloyd's, the world's leading insurance and reinsurance marketplace, said it was committed to accelerating its transition towards "a more sustainable insurance and reinsurance marketplace".
"We are actively involved in constructive engagement on the issue of climate change and continue to explore the ways in which Lloyd’s can support a responsible transition," a Lloyd’s spokesman said.
Some major Lloyd's insurers have ruled out insuring the Carmichael mine, from which Adani plans to start producing 10 million tonnes of coal per year from 2021.
Lloyd's has asked members to stop providing new insurance cover for thermal coal, oil sands, or new Arctic energy exploration from Jan. 1, 2022, with a target of 2030 to phase out the renewal of existing cover.
Extinction Rebellion wants to prompt a wider revolt against the world's political, economic and social structures to avert the worst scenarios of devastation outlined by scientists studying climate change.
Activists from the group smashed the window frontage of HSBC's headquarters in Canary Wharf on Thursday and two weeks ago shattered windows at Barclays.
On Thursday, the United States and other major industrialised countries hiked their targets for slashing greenhouse gas emissions at an Earth Day global climate summit hosted by President Joe Biden.