Police in five cities are seeking an armed robber believed to be seen here holding up a Cambridge Street convenience store Monday night.

After nearly two dozen armed robberies that have him wanted by police in five cities, a man found a little more resistance than expected from a convenience store clerk Monday on Cambridge Street in Cambridge.

The robber, described as a dark-skinned black man aged 30 to 35, standing about 5 feet, 10 inches tall and bearing a thin mustache and usually wearing hoodies and sunglasses, was the subject of press releases Nov. 1 from police who wanted an end to a spree then described as hitting “at least 21 convenience stores” in Cambridge, Chelsea, East Boston, Everett, Jamaica Plain and Lynn. The robber was on the job in Cambridge Oct. 28 and in Everett only three days later.

“We are very concerned that he is going to keep doing this until he is arrested,” said Brian Kyes, chief of Chelsea police.

Maybe so. The same robber struck again at 9:04 p.m. Monday in Inman Square, police said, for a two-minute incident that sounds like it unfolded less than predictably.

According to the store employee who was robbed and surveillance video, the robber walked in, wearing a dark gray or light black hooded sweatshirt, dark gray or light black jeans, white sneakers and the usual dark sunglasses and pulled up the bottom of his sweatshirt, exposing the black handle of a handgun, possibly a revolver.

“Can I help you?” asked the apparently unimpressed clerk, according to the police report.

“Give me everything you got,” the robber said. “Open up the register.”

But the clerk resisted, telling the robber he couldn’t open the register unless someone bought something.

That caused an argument, the clerk told police. Rather than just, say, buying a pack of gum so he could take all the money in the cash register, the robber demanded the clerk just press the “open-drawer key.”

Instead, the clerk asked the robber to show him his gun, to which the robber responded, “Just give me the money.”

He got away with about $100, police said, turning right out the doors of the convenience store and heading toward Hampshire Street.

“Luckily no one has been injured,” said Robert Haas, commissioner of Cambridge’s police department, in the Nov. 1 press release. “We need the public’s help to identify and apprehend this suspect before these robberies turn violent.”

Anyone with information is asked to call Cambridge Police at (617) 349-3300. Anonymous reports can be left at (617) 349-3359; sent by text message to 84741 (begin your text with TIP650, then type your message); or by email by visiting CambridgePolice.org/Tips.