China is on high alert as it deals with new security threats on the opening day of the Olympics.

An Air China plane that departed from Japan had to make an emergency return Friday after the airline received an e-mailed bomb threat.

Japanese authorities say the e-mail threatened to bring down the flight on the site of the Beijing Olympics.  The flight was bound for the southwestern city of Chongqing when it was forced to return to Tokyo's Narita airport.

In Hong Kong, local media reports say police cordoned off a busy train station to investigate a suspicious package near a giant screen set up to show the Olympics opening ceremony.

A convoy of armored personnel carriers and trucks carrying paramilitary officers patrol the streets of Yining, western China's Xinjiang province, 08 Aug 2008

Chinese police tightened security in the remote region of Xinjiang, where an Islamic group threatened to attack buses, trains and planes during the Olympics.

Police shut down a popular bazaar in the region's city of Kashgar and stepped up control on religious figures and alleged trouble makers.

The order by the Kashgar government followed an attack earlier this week that killed 16 police officers and a new video threat by separatists in Xinjiang to attack the Games.

Two U.S. groups that analyze extremist messages - SITE Intelligence group and the IntelCenter -- say the Turkistan Islamic Party is responsible for the new video tape that was released on Thursday.  

The video features graphics of a burning Olympics logo and an explosion over an Olympic venue.  Chinese authorities have not commented on the validity of the tape or the threat.

Some information for this report was provided by AFP, AP and Reuters.