The workers, who are members of PCS, are employed by the Metropolitan Police.
The strike comes a day after the Met imposed rota changes on staff despite 95% of the guards boycotting a ballot on whether to accept it because of the risks the working arrangements pose to their health and well-being.
PCS officials say staff accepted two years ago that shifts had to change, as overtime is currently paid on Friday nights because there are not enough volunteers.
Negotiators accuse senior officials at the Met of repeatedly “moving the goalposts” when the union presented a range of proposals to them.
The latest alternative drawn up by staff would mean a longer run of night shifts but with more days off in between, while being fully compliant with European working time and health and safety regulations.
In light of the Met’s refusal to re-enter talks, 74% of the union’s members voted for a strike and 91.5% voted for other forms of industrial action, on a 44% turnout.
PCS general secretary Mark Serwotka said: “Anyone who works in parliament or has ever visited will have been greeted by these friendly, dedicated security staff who have been treated appallingly by their Met Police bosses.
“It is outrageous that the Met is trying to impose new ways of working on staff who have bent over backwards to come up with suitable alternatives, only to be ignored.”
Officials say the 24-hour walkout – which began at 6am – will seriously disrupt security arrangements in parliament, with police officers expected to be drafted in to provide cover at great expense.
The stoppage will be followed by a three-day work to rule and overtime ban.