The only ride untended rental scooters can count on in Boston is a quick one: to the city’s tow lot.

Mayor Martin J. Walsh is warning electric scooter rental companies, which are zooming now into cities across the country, that their hipster rides will be confiscated if they try to start service in the Hub without permission.

“They can’t just show up here, there has to be some regulation and some notification of what’s going to happen,” Walsh said yesterday. “If they drop them off here, we’re going to pick them up off the street and they can come pick them up at the tow yard.”

On Friday, California-based Bird abruptly announced the start of their scooter rental service in Somerville and Cambridge, a move that took both cities by surprise.

“We learned about it by the press calling us about it on Friday,” said Denise Taylor, a spokeswoman for the city of Somerville.

The Bird scooters run on electric motors, and top out at 15 miles per hour, according to the company. Riders unlock a scooter using their phones, and pay $1 plus 15 cents per minute to use it. Because they don’t have docks like Boston’s Blue Bikes bike-share, the scooters can be left on the sidewalk once a rider gets to their destination.

Scooters from Bird and other companies have come into cities across the country, largely without permission from local governments. Bird says it is in more than 20 cities. They have been criticized for being dangerous and illegal.

In a statement, a Bird spokesperson said the company expects to work with Somerville and Cambridge.

“The cities of Cambridge and Somerville share Bird’s vision of building communities with fewer cars, less traffic, and reduced carbon emissions,” the statement said. “We have reached out and look forward to working closely with local leaders and officials to develop a framework that permits affordable and convenient transportation options.”

Bird declined to comment on the possibility of expanding to Boston, and would not say how many scooters it has in Cambridge and Somerville.

Walsh said the city has had preliminary discussions with Bird, but he’s not aware of any plans to expand here.

“It could be a real hazard here in Boston,” Walsh said. “We’d want to have a real good vetting process to see if in fact we want something like this.”

Many in Harvard Square were curious to see the new scooters zipping by yesterday.

“I find them fascinating,” said Rachel Richter, 27 of Cambridge. “I’d like to hop on one. It is a good way to get around. I think some people might find it as a more convenient way to travel but you’re more exposed to the elements.”

Others said safety should come first, especially when it comes to riding in traffic and wearing helmets. Bird advises people to wear helmets and follow traffic rules. A driver’s license is also required to be able to hop on a scooter.

“I was wondering why so many people were using them,” said Angela Miller, 50 of Belmont. “I’m not opposed to it. I prefer to walk. It doesn’t seem harmful to me as long as people wear helmets. Hopefully, they’ll be in areas where people aren’t moving so fast.”

Chris Dempsey, director of Transportation for Massachusetts, said private transportation options always work better when they are working hand in hand with local officials, but said scooters represent the possibility of a new way to get around in the Boston area.

“We know that transportation in Greater Boston is not working,” Dempsey said. “We need to be looking for ways to give people more and better options.”