UPDATED, 4:25 p.m.: Firefighters have halted their search after not finding anybody in the water, according to scanner reports.
UPDATED, 3:48 p.m. with information from Waltham Fire Dispatch center Twitter: A State Police helicopter has been called to the scene to assist with the search.
Waltham firefighters are conducting a water rescue on the Trapelo Road side of the Cambridge Reservoir, according to Twitter user Matthew Carter (@matredsoxfan2) and scanner reports.
Crews are using a boat to conduct a search of the water, according to scanner reports.
A Waltham fire deputy chief and ambulance are on the scene along with other first reponders, according to scanner reports. Waltham Police are also on the scene.
Stay tuned to Waltham Patch for updates.