The protest, which blocked roads in the Knightsbridge area of central London, was organised by the union that represents hospitality workers in the store as part of a row over tips.
The United Voices of the World (UVW) union complained that Harrods was keeping 75% of the cash left by diners.
Harrods said it was reviewing its service charge system
The Metropolitan Police said an 18-year-old man was arrested on suspicion of setting off a flare.
A 51-year-old woman was also arrested on suspicion of criminal damage and assaulting a police officer.
UVW's general secretary, Petros Elia, told the BBC Harrods was keeping up to Â£2.5m a year in tips from its hospitality staff.
A Harrods spokeswoman said the store employed more than 450 workers in its 16 restaurants, all of whom were paid "fairly and above national living wage".
Harrods said the employees would be given details of the new system, as soon as a review was complete.
There is currently no law requiring restaurants to hand over all the tips to their staff.
However the Business Secretary Sajid Javid has said gratuities should go to the people they were intended for.
In May 2016 the government highlighted a range of options to improve the tipping system, including the idea of legislation.
Industry guidelines suggest restaurants can keep a proportion of tips, to cover their costs.
Two people were arrested after a smoke bomb was let off during a protest outside Harrods department store.