TOKYO -- Jin Saburi was drinking coffee at a Tokyo Denny's when an urgent message hit his cellphone: "Shion, level 12, requests a rescue. Will you respond?"

The "rescue" wasn't real -- it was part of the fantasy game Mr. Saburi was playing on his mobile phone. But the plea for help was genuine: Shion was another gamer somewhere in Japan playing that same game -- one of thousands of Japanese playing each other, in real time, over the cellular network.

"It's nice to have the feeling that other people are playing along with you," says the 33-year-old Mr. Saburi. He went on to rescue Shion and so many other players that day that he ended up having a second meal at Denny's.

Mr. Saburi and his pals are providing an early glimpse into one of the most eagerly anticipated but challenging areas of the videogame world: online play on the go, using light, portable devices.

Although the market is still small and fraught with technical hurdles, game makers see networked portable devices as a vital way to expand online gaming beyond the home, and make their titles for handheld consoles and cellphones more enticing. Interest has soared this year, sparked in part by the introduction of new, WiFi-capable handheld game machines from Sony Corp. and Nintendo Co. Both companies will be talking up new networked titles for their handhelds this week at the Electronic Entertainment Expo, or E3, videogame show in Los Angeles.

For gamers, the networked portable devices can offer the same sense of bonding and human contact that traditional online games do, but without being stuck in front of a computer.

So far, almost all the action is on cellphones. In Japan, where global videogame trends usually start, the cellphone-game market was valued at $280 million last year, according to data tracker Enterbrain Inc. And almost every title had some network feature.

Japan's cellphone gamers compare scores in football matches and mate each others' thoroughbreds in racehorse games. Mobile carriers have also teamed up with game makers like Tokyo-based G-mode Co. to offer head-to-head play for everything from Othello to a Japanese version of paper-scissors-stone.

Now, networked mobile gaming, while still small, is also picking up in the U.S. Carriers such as Sprint Corp. have sites where users can download multiplayer versions of single-player classics like cellphone poker. San Mateo, California-based Sorrent offers the Daily Puzzle, where users can download a series of brainteasers daily, and get statistics showing how their answers compared with those of other players.

Handheld-console makers are making their first real push into networked play this year, in a move that industry watchers say could give the market a real boost. This month, Sony's South Korean videogame unit put out a car-racing game that lets PlayStation Portable players battle each other over a broadband network. Sony says more such games are coming up. Nintendo last week said it would create online services for its DS handheld with U.S. Internet game portal IGN Entertainment. At E3, Nintendo will be showing its first networked games for the DS, including a version of Animal Crossing that will let players visit each others' virtual animal villages via the Internet.

To be sure, both handheld game machines and cellphones face obstacles with gaming on-the-run. Handhelds have plenty of fun games but spotty access to the network, because there are still relatively few "hot spots" where players can go wirelessly online. Mobile phones, meanwhile, have easy access to a network, as well as a huge pool of potential customers. But they transmit data too slowly for fast-moving, head-to-head action games, making them unsuitable for certain kinds of games.

Another hurdle is gathering a big enough pool of potential players so that even at midnight you can find a partner for, say, a game of cellphone billiards, says Takeshi Miyaji, G-mode's president. G-mode mediates some 200,000 head-to-head plays a day; Mr. Miyaji says that kind of volume is essential. Typically, G-mode gamers play once or twice a day, which adds up to about $30 per month in data-transmission fees.

Square Enix Co., which made a version of its hit Final Fantasy role-playing game for the cellphone, takes a different approach to encourage more people to play online. Its game requires some team play over the network: If you're captured by enemies, you can't continue playing until a fellow gamer who is online has rescued you. But because requests for aid go out broadly over the network, chances are that someone will be free to help.

"It's not heavy-duty online, it's online lite," says Masahiro Hora, general manager of Square Enix's mobile business division. Square Enix is hoping to bring that Final Fantasy cellphone game to the U.S., and will unveil another networked game for mobiles at E3.

The growth of a community of gamers who play each other via cellphone is even spawning its own subculture. Satoru Kojima, a fan of G-mode's multiplayer Tetris game, decided two years ago that he really wanted to contact a player nicknamed Dona-chan who sometimes beat him in competition. Because there was no way to communicate directly, Mr. Kojima created his own cellphone Web site dedicated to Tetris chat, which he advertised through messages posted on the G-mode high-scorers' list.

Mr. Kojima did manage to contact Dona-chan, and the two now chat regularly on his cellphone bulletin board. And Mr. Kojima's page has drawn postings from nearly 70 cellphone Tetris fans. That feeling of easy contact, says Mr. Kojima, is part of the fun playing online, on the phone: "Playing with people is absolutely more fun than with a machine."