Hackers are targeting users of the mobile-phone messaging application Line, according to the company and Japanese police.

Tokyo-based Line Corp. said that in late May it received a report of a hacking case for the first time since introducing the app in June 2011. The popular app, which enables users to make free voice calls and send free messages, has more than 480 million registered users, the company says.

By mid-June, the number of reported cases rose past 300, a Line spokeswoman said. In a typical scam, hackers pretend to be the owner of the account and ask friends to buy prepaid cards on their behalf. Police in Tokyo and Osaka have issued warnings to users, telling them to keep their usernames and passwords secret.

Police in Osaka said 23 reports of hacking have been filed as of this week with total monetary losses of ¥654,000, or about $6,500. In Shiga Prefecture, police said two people reported they were cheated out of ¥220,000 on July 4.

Line warns against users setting the same IDs and passwords for multiple web services and recommends that users pick more complicated passwords that combine letters, numbers and symbols in a random string. The Line spokeswoman said user information didn't leak from the company's servers. She said hackers couldn't steal personal information such as phone numbers just by looking at a friend list.

Users who believe their account has been hacked can ask the company to delete the account through an online contact form.

Wall Street Journal reporter Megumi Fujikawa had her own Line account hacked recently and describes the experience as follows:

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"Did you ask me on Line to buy some kind of prepaid cards for you?" my husband asked one evening.

I checked my iPhone and found many similar messages from friends via email, SMS and Facebook.

I rushed to open my Line account but kept failing to log in. All I got was this warning: "You cannot use this account. Since this account has been logged on in a different location, all information saved in this terminal will be deleted."

That's when I realized my Line account had been hacked.

After sneaking into my account, the hacker changed the registered email address and password so I lost control of my account completely. After having some conversations with my friends, the hacker disappeared and deleted the account.

A friend later passed on the conversation she had in Japanese with the hacker pretending to be me.

Hacker: What are you doing? Are you busy? Can you help me?

Friend: What do you need?

Hacker: Can you help me buy prepaid cards for web money at your nearest convenience store?

Friend: OK, how much?

Hacker: Please buy four 20,000-point cards. I can't go right now, so I'll give you money tomorrow.

Friend: Got it.

Hacker: Please send me a picture of the serial numbers after you buy them.

Friend: Sorry, my phone's camera is broken.

Hacker: Could you do something to help me? Please. This is urgent. Send it to me ASAP. Please...

My friend didn't actually buy the cards, saying she recognized that the messages were probably coming from a scam artist. Ultimately I didn't lose any money. But I decided against opening a new Line account for now.