About half of those crimes were reported near the California Avenue Caltrain Station and the Palo Alto station on University Avenue. The others were spread throughout the city.
Police believe Jeffrey Smith, the man arrested Tuesday, matches the description of the suspect in at least a few of the crimes.
"I believe he might be responsible for four or five of them, right off the top of my head," Ryan said.
The first of the two robberies on Tuesday was reported around 7:20 p.m. in the 100 block of Bryant Street.
A woman walking home was attacked from behind by a man who tried to grab her purse. She was pushed to the ground and screamed as she fell, police said.
Witnesses told police the suspect ran to a tan imported sedan and fled the area without the purse. They provided police with the car's license plate number.
About 25 minutes later, Menlo Park police responded to the intersection of Oak Grove Avenue and Middlefield Road where another woman said she had been robbed of her purse.
Police from both cities searched the streets for the vehicle from the Palo Alto incident, and found it at about 9:10 p.m. on West Bayshore Road in East Palo Alto.
Smith was detained after being found in possession of the purse taken in the Menlo Park robbery. He was positively identified by a witness and arrested by Menlo Park police.
Ryan said the tan sedan seen in the attempted robbery Tuesday night matches the description of a car seen in at least a couple of the other 18 robberies and burglaries.
"There was a small, four-door sedan described at least twice that I can think of," Ryan said. "This would match that."
Police suspect several individuals or groups may be responsible for the city's robberies and officers have been out in full force to catch the culprits.
Efforts have included doubling patrol units, moving officers off other assignments and into the downtown area, and using undercover operations and plainclothes officers, Ryan said.
At a recent community meeting on the robberies, Palo Alto Police Chief Lynne Johnson sparked controversy by making statements some said show that the department is guilty of racial profiling.
Suspects in many of the robberies have been described as black, and at the Oct. 30 meeting, Johnson said she had asked officers to make "consensual contact" with black men seen in locations where robberies have occurred.
The comments provoked a backlash from residents, church leaders, Stanford University students and others.
Johnson has since apologized, though community groups in Palo Alto and neighboring East Palo Alto, which has traditionally had a larger black population than Palo Alto, have expressed continued discontent and held a protest march to Palo Alto City Hall.
While the debate surrounding Johnson's comments has not been resolved, police will continue to investigate the crimes for which Smith was arrested and hope to solve a few more of the robberies.
"There is just a lot of leg work to be done," Ryan said.
Witnesses and victims will likely be contacted to make a possible identification, Ryan said. He anticipated at least a week's worth of police work before the case can be forwarded to the district attorney's office.
"We're going to be working on it diligently," he said.