Unite Here Local 2, which represents about 700 room cleaners, dishwashers, clerks and other workers at the two hotels, began the strike at 4 a.m. today outside the Hyatt Regency at 5 Embarcadero Center and the Grand Hyatt at 345 Stockton St., union spokeswoman Julia Wong said.
The strike is also taking place at Hyatt hotels in Chicago, Honolulu and Los Angeles, Wong said.
The Hyatt workers' labor contract expired in 2009 and negotiations for a new contract are still under way.
Wong called Hyatt "the most abusive company in the hotel industry," alleging that the hotel chain overworks employees, resulting in frequent workplace injuries.
She said about 100 workers were picketing outside the Hyatt Regency this morning, and that the picket lines would remain there for all seven days of the strike.
Hyatt spokesman Pete Hillan said guests at the hotels, which are mostly booked through the next week, will still receive the same level of service that they normally would.
"For us, the real issue is the disrespect Local 2 leadership is showing for its own members," Hillan said.
He said Hyatt management in March offered the union a contract similar to ones they signed with other hotel chains in San Francisco, but that the union is now making new demands that he says are "a ruse for the membership-building they're trying to do nationally."
He said, "Local 2 leadership is just taking a week's worth of pay out of the hands of their own members."