Western countries have stepped up security measures following a series of attacks on London transport systems.
Washington has deployed sniffer-dogs
The US has raised its terrorist threat warning level from elevated to high across its transport network as Canada beefed up checks in its biggest city.
France, Spain and the Netherlands have raised national levels of security.
Belgium bolstered measures at train and underground stations, border points and ports into the country, which houses a number of EU institutions and Nato.
Dutch Prime Minister Jan Peter Balkenende said security would be stepped up at the Dutch borders and around British sites in the Netherlands.
Elsewhere in Europe:
- Germany bolsters security on national railway and Berlin underground system and train stations
- Italy places all airports on maximum alert
- Poland boosts security at train and bus stations, airports and Warsaw underground
Other countries stepping up security measures include Russia, the Czech Republic, Romania and Hungary.
US homeland security chief Michael Chertoff said the move to raise its terrorist threat, along with other security measures, was a "commonsense" response to the bombings.
Police with machine guns and sniffer dogs were swiftly deployed on streets and subways in cities like Washington and New York.
Earlier President George W Bush - at the G8 summit in Scotland - held a video conference with national and homeland security officials, urging them to be "extra vigilant" as the US public headed to work.
The nationwide US rail system, Amtrak, said it had raised its security alert level.
Other major cities including Los Angeles and Chicago heightened security on transit systems.
Canadian Prime Minister Paul Martin has also asked his national security adviser to "monitor the situation".
Officials in Toronto urged underground crews and bus operators to remain vigilant.