More than 200 residents called city emergency lines between 5 and 6:30 p.m. Wednesday to report a mysterious "noxious odor" in the northern part of Palo Alto, Sgt. Sandra Brown said.
Many others calls came from Menlo Park and East Palo Alto residents, officials reported.
City fire and utility crews have not been able to find the source of the odor. Neither city utilities nor Pacific Gas & Electric have reported any broken lines in the area, Brown said.
"We don't know where it's coming from," she added.
Two residents said the odor smelled like natural gas.
The odor was also reported from as far south as Midtown and in southern Menlo Park and East Palo Alto.
The weather station at Palo Alto Airport registered a 6-mile-per-hour breeze from the northeast shortly after 7 p.m.
At 7:25 p.m., Palo Alto activated its new Community Alert and Notification System to alert residents of the odor. Emergency coordinator Sheryl Contois said San Mateo County officials also reported receiving numerous calls from Menlo Park and East Palo Alto residents.
The U.S. Coast Guard duty officer said there were no incidents reported from the bay that could account for such an odor.
Officer Graciela Castillo of the California Highway Patrol Redwood City office reported that no possible source of the smell, such as a truck accident or other incident, has been reported.
Brown said the emergency-notification system sent approximately 22,000 messages to residents, and 80 percent were answered by someone or an answering machine. To receive such alerts, including on cell phones, residents and businesses must register for Connect-CTY on the ity's Web site at http://www.cityofpaloalto.org , Brown said.