Streep plays political activist Emmeline Pankhurst opposite Mulligan's Maud - a fellow activist in the feminist movement - in the project from Pathé, Film4 and the BFI Film Fund.
The stars were seen in period costume as they filmed in the capital, with Mulligan sipping from a mug in between takes.
Helena Bonham Carter was also pictured on location, although it is still unknown what character the actress is portraying.
The historical film is directed by Sarah Gavron and written by Abi Morgan, who previously wrote Margaret Thatcher biopic The Iron Lady, also starring Streep.
Based on true events, the movie will and focus on suffragettes who lobbied for women's right to vote.
Romola Garai, Anne-Marie Duff, Brendan Gleeson, Ben Whishaw and Samuel West will also star in Suffragette, which is due to be released in 2015.