CHENNAI - Police in India have released dozens of those detained for blocking a key highway in a protest against food poisoning at a Foxconn unit, the country's second instance of unrest at an Apple Inc supplier factory in a year.

India is among the countries, such as Mexico and Vietnam, that are becoming increasingly important to contract manufacturers supplying American brands as they try to minimise the impact of the trade war between China and the United States.

The highway was blocked for hours in the southern city of Chennai in Tamil Nadu state, home to a plant where the Taiwan contract manufacturer, formally known as Hon Hai Precision Industry Co Ltd, began assembling the iPhone 12 this year. 

It was not immediately clear if production was disrupted by the protests sparked by last week's food poisoning incident that led to 150 employees being admitted to hospital. 

Foxconn declined to comment. Apple did not immediately respond to an email seeking comment.

A police official said plant workers and their relatives were among those who blocked the highway linking Chennai to India's technology hub of Bengaluru.

While the nearly 70 women detained over Saturday's protest were released the following day, some of the 22 men held were arrested, added the official, who sought anonymity as he was not authorised to speak to media on the issue.

Most of Foxconn's workers in India are women.

In December last year, thousands of contract workers at a factory owned by Apple supplier Wistron Corp 3231.TW destroyed equipment and vehicles over the alleged non-payment of wages, causing damages estimated at $60 million.  

(Reporting by Sudarshan Varadhan and Sayantani Ghosh; Editing by Michael Perry and Clarence Fernandez) ((rajendra.jadhav@thomsonreuters.com; +91-22-68414378 ; Reuters Messaging: rajendra.jadhav.thomsonreuters.com@reuters.net))