Flashing back to the Sept. 11, 2001, attacks in New York and the train bombings in Madrid last year, countries around the world stepped up security, fearing more terror could be in the offing.
, the U.S. Homeland Security Department asked authorities in major cities for heightened vigilance of major transport systems.
Spain, also bitterly familiar with terror, put its security forces on maximum alert, posting army and police units to watch over airports, train stations and shopping centers. The government of Prime Minister Jose Luis Rodriguez Zapatero offered its "unconditional help to chase the criminals who perpetrated such a repugnant attack." The killed 191 people.
A similar response came from France, the object of attacks in the 1990s. Prime Minister Dominique de Villepin ordered the alert level raised a notch and promised Britain the "immediate, full and complete collaboration" of French intelligence.
The French terror alert is now at red, the second-highest level in France's four-step system, reports CBS News Correspondent Elaine Cobbe. That means more police and soldiers in train stations, airports and on the streets.
"It is a drama for Great Britain. It is a drama for all of Europe," Villepin said, recalling the Madrid bombings.
"What happened today demonstrates yet again that we are doing too little to unite our efforts in the most effective way in the battle against terrorism," said Russian President Vladimir Putin at the G8 summit.
He has in the past criticized other countries for underestimating the ties of Chechen separatist rebels in Russia to other terrorists.
German Chancellor Gerhard Schroeder stressed the need for fighting terrorism "with all the means at our disposal." Security was raised for the subway, bus and local train service, reports CBS News' Peter Bild, and the overall alert was raised to 2 out of three levels.