Japan has stepped up security around its nuclear plants and on its rail and subway networks following the attacks in London on Thursday.
Prime Minister Junichiro Koizumi is a staunch US ally
Extra measures have been introduced to help protect commuters on lines already subject to stringent checks.
A government spokesman said Japan, which is a G8 nation and has troops stationed in Iraq, was on high alert.
But Prime Minister Junichiro Koizumi said there were no plans to withdraw Japanese forces from Iraq.
The BBC's Chris Hogg in Tokyo says the Japanese authorities fear that because the capital's metro alone carries as many as six million passengers a day it is particularly vulnerable to terrorist attack.
The new measures include the removal of rubbish bins that had only been recently been re-installed after previous security scares, and more police on duty at major stations.
But security experts say the authorities have their hands tied. Legal limits on surveillance activities make it more difficult to gather detailed intelligence in Japan than in some other countries.
A high-ranking police official is reported to have warned that despite the sarin gas attack on the Tokyo subway 10 years ago, Japanese people are less wary of terrorism than they need to be.
Unless the public becomes more vigilant, he said, it will be impossible to prevent a terrorist attack.
A previously unknown group said it carried out the blasts as revenge for the "massacres" Britain was committing in Iraq and Afghanistan.
Some 500 Japanese troops are currently in the southern Iraqi city of Samawa.
But the attacks and Japan's operation in Iraq "should not be directly linked", Mr Koizumi said, speaking at the G8 summit in Gleneagles.
He did stress the need for Japan to take steps to prevent attacks on its own soil.
"No country can say that there is no possibility. I think we need to continue to take sufficient steps against terrorism. You cannot tell when or where it could happen," Mr Koizumi said, the Reuters news agency reported.
In another measure, Japan issued a travel warning to its citizens in London, urging them to avoid central London.
South Korea, the other key Asian member of the US-led coalition with troops in Iraq, said the London bombings were "cruel and inhumane crimes".
"Our government declares an intention once again to work actively together with the international community to root out terrorism," it said in a statement, the AFP news agency reported.