At least 31 people were killed and 130 wounded Monday in a suicide blast at Domodedovo airport in Moscow, Russian Health Ministry officials said.
Smoke wafted out of the baggage claim area and people were seen running out of the emergency exits at Russia's busiest airport, local media reported.
Russian officials branded the bombing a terrorist attack and President Dmitry Medvedev vowed Monday that those behind the explosion would be "tracked down and punished."
The president called for all transport hubs across the country to be on high alert Monday. The order, announced on Russian television, comes into immediate effect and affects all airports and train stations.
Medvedev also criticized apparent lax security at Domodedevo. He has meanwhile delayed his own departure for the World Economic Forum in Switzerland.
Mark Green, a British Airways passenger who had just arrived at the airport, told BBC television he heard the huge explosion as he was leaving the terminal.
"Literally, it shook you," he said. "As we were putting the bags in the car a lot of alarms ... were going off and people started flowing out of the terminal, some of whom were covered in blood."
One gentleman had a pair of jeans on that was ripped and his thigh from his groin to his knee was covered in blood, he added. Green said thousands of people were in the terminal at the time of the blast.
The Russkaya Sluzhba Novostei radio station cited a traveler, identified as Viktor, as saying he heard the bang outside the airport, where he was waiting for a car.
"There was an explosion, a bang. Then I saw a policeman covered in fragments of flesh and all bloody. He was shouting 'I've survived! I've survived!'"
Domodedovo is generally regarded as Moscow's most up-to-date airport, but its security procedures have been called into question.
In 2004, two suicide bombers were able to board planes at Domodedovo by buying tickets illegally from airport personnel. The bombers blew themselves up in mid-air, killing all 90 people aboard the two flights.
Russia's rouble-dominated stock market MICEX fell by nearly 2 percent following the blast, which ripped through the baggage claim area of the airport at around 1330 GMT.
Moscow suffered its worst attack in six years in March 2010 when two female suicide bombers from Russia's volatile Dagestan region set off explosives in the metro, killing 40 people.
The Kremlin is struggling to contain an Islamist insurgency in the mainly Muslim North Caucasus, and rebels have repeatedly vowed they will take their battle to the Russian heartland.
Israel halted its flights from Tel Aviv to Moscow following the attack.