OSAKA — Santen Pharmaceutical Co., the nation’s leading maker of eyedrops, has begun recalling its products after an extortionist threatened to contaminate its eyedrops with an undisclosed substance unless it pays some 30 million yen in cash.

Takakazu Morita, president of the Osaka-based company, told reporters Thursday that Santen has started recalling all of its 24 types of eyedrops for use by the general public and asked that the public temporarily refrain from buying its products.

Telephones at Santen’s headquarters were flooded with calls from concerned retailers and consumers Friday, and one company official said that while no one was counting, the number of calls may have totaled 1,000 or so by midmorning.

In the letter, which Santen received Wednesday, the writer threatened to randomly contaminate the eyedrops unless the company pays the money.

Investigative sources said Friday that the letter consisted of two handwritten sheets of A4-size paper. The characters were written in a broken style, apparently in an effort to disguise the writer’s penmanship.

The sender’s name and address were on the envelope, but no such person or place exists, according to the sources. The postmark, although unclear, indicated the letter was posted in Osaka on Tuesday, they said.

The letter contained no demand besides money and there were no details regarding why the sender had targeted the firm, the sources said. Police said they are investigating a variety of angles and have not ruled out the possibility that it is merely a malicious prank.

The letter named the substance that would be mixed into the eyedrops, but investigators, without identifying the substance, said there is little likelihood that it would be very harmful to humans.

Company officials said the firm expects all of its eyedrops to be collected from retailers in about two days. It will take about 10 days to withdraw them from the entire domestic market.

The company will also stop running television commercials during the recall period. Products purchased before the letter was mailed are believed to be safe.

A total of about 2.5 million items will be recalled, costing the company about 300 million yen, they said.

Santen soon plans to change the packaging of the eyedrops jto prevent them from being tampered with.

The company said it alerted police as soon as it received the letter, which also included a product apparently mixed with the contaminant.

Santen has not paid any money to the extorter and has no intention of doing so, Morita said.