Japan ordered riot police to patrol airports and guard nuclear plants and beefed up security at other key facilities nationwide, in a precaution against terror attacks as the country deploys troops in Iraq.
About 650 vital facilities, including US military bases, were put under increased surveillance, the Yomiuri newspaper and other media reported. An agency spokesman could not be reached on Saturday to confirm that figure.
Authorities would not say if they had new information about a terrorist threat or reveal how many police officers would be mobilised in the build-up.
The National Police Agency ordered local law enforcement authorities on Friday to raise security around possible terrorist targets to its highest level since the US-led invasion of Iraq in March 2003.
The alert sent a shiver through global financial markets, knocking the Japanese yen to a 10-week low against the US dollar on Friday.
It was issued amid concern about terrorist retribution as Japanese troops joined the US-led coalition in Iraq on a non-combat mission to rebuild its war-shattered infrastructure.