Casablanca, Morocco (CNN) -- A series of explosions that killed 15 people when they erupted Thursday morning in a cafe in the city of Marrakech was an act of terrorism, Morocco's interior minister said.
"We will investigate this act of terrorism and find those responsible ... and their nationality," Taib Cherqaoui told reporters about the attack, in which at least 22 others were wounded. "We condemn it as a criminal act."
In an address on state-run television, Cherqaoui and King Mohamed VI said the fatalities included six French nationals, five Moroccans, and four others whose nationalities were not divulged. CNN has learned that, of the four remaining fatalities, two were Russian and one was British.
The incident occurred around 11 a.m. at Cafe Argana in Marrakech's old city, which is designated by the United Nation's cultural arm as a World Heritage Site. Tourists flock to it in high numbers this time of the year and it is usually packed with stalls, story-tellers and snake-charmers.
World leaders moved quickly to condemn the incident. U.N. Secretary-General Ban Ki-moon was "appalled," according to a statement from his office. "He reiterates his firm rejection of the use of indiscriminate violence against innocent civilians and maintains that no political objective justifies or is served by such heinous acts."
U.S. Secretary of State Hillary Clinton said the United States "condemns in the strongest terms today's terrorist attack," adding, "Acts of terrorism must not be tolerated wherever and whenever they occur."
French Foreign Minister Alain Juppe condemned what he called a "barbarian terrorist attack, which nothing can justify."
"Such an awful act can only bring indignation and unreserved condemnation. All light must be shed on this revolting crime, officials prosecuted, tried and punished," said Juppe, who said French authorities will provide assistance to French nationals in Marrakech.
"We were walking around the souks, right around the corner from Cafe Argana," said a German tourist who was about 50 meters from the blast when it occurred. "We heard a gigantic boom, and everyone immediately starting running towards the square to see what happened."
The woman, who didn't want her name used, told CNN the top floor and terrace of the cafe were "ripped apart" by the blast, which caused hundreds of people to run from the area.
It was not immediately known whether the attack was linked to unrest across the Arab world or militant activity, although there have been protests in Morocco lately.
The blast comes four days after thousands of Moroccans held a peaceful demonstration nationwide, calling Sunday for a radical overhaul of the country's governance before a new constitution is unveiled in June by King Mohammed VI.
The march was organized by the Facebook youth movement Fevrier 20. The group said its members would not accept the draft constitution because it was written by the king's own people. It denounced his decision to refer the new constitution to a committee he appointed.
King Mohammed announced last month he would give up some of his wide-scale powers and make the judiciary independent -- the latter a particularly hot subject in Morocco.
Calls for an end to political detention and questions about the king's personal business activities were also on protesters' banners. There was visible resentment at the royal family's business operations, controlled by its holding company SNI. There were also groups protesting about the prices of basic household items.
Since the Casablanca attacks of 2003, in which 33 bystanders and 12 suicide bombers were killed, the Moroccan government has impressed international officials for breaking up Islamic terrorist cells.
For example, the Madrid train bombings in 2004, in which 191 people died, were assisted by a Moroccan group of terrorists -- 19 of whom are serving sentences in Moroccan prisons.
CNN's Mitra Mobasherat and Journalist Martin Jay contributed to this report