Pope Francis has voiced his "strongest condemnation" of the "horrible attack" on a French satirical newspaper that published insulting cartoons of Islam's Prophet Mohammed, and he urged opposition to all hatred and violence.

"Whatever may be the motivation, homicidal violence is abominable. It is never justified: the life and dignity of all must be firmly guaranteed and guarded; any instigation to hate refuted; and respect for the other cultivated," Holy See press office director Father Federico Lombardi said Jan. 7.

He said Pope Francis "joins the prayers of the suffering and wounded, and of the families of the dead." The Pope also expressed his closeness, spiritual solidarity and support for all those who "continue their constant efforts for peace, justice and law" in order to "heal in their depth the sources and causes of hate."

The Pope "exhorts everyone to oppose, with every means, the spread of hatred and of every kind of violence, both physical and moral, with destroys human life, violates the dignity of the person, and radically undermines the strong foundation of peaceful coexistence among persons and peoples, notwithstanding differences of nationality, of religion, and of culture," Fr. Lombardi said.

On Wednesday gunmen armed with a Kalashnikov and a rocket launcher attacked the Paris office of the newspaper Charlie Hebdo. They killed at least 12, including two police officers, and critically wounded four.

The attackers hijacked a car and sped away, shooting at police and running over a pedestrian, Agence France Presse reports.

According to police, witnesses said the attackers shouted phrases including "we have avenged the prophet" and "Allahu akbar," an Arabic phrase meaning "God is greatest."

The newspaper has a history of publishing provocative cartoons about Mohammed. In February 2006 it reprinted cartoons from the Danish newspaper Jyllands-Posten that provoked outrage among many Muslims; the newspaper's offices were firebombed in November 2011 after publishing one cartoon of Mohammed.

In September 2012 the newspaper published cartoons showing Mohammed naked. The cartoon came at a time of violent protests over a low-budget film made in the U.S. that was insulting towards the founder of Islam.

The latest edition of Charlie Hebdo featured a front-page story on French author Michel Houellebecq, whose controversial new novel "Submission" portrays a France ruled by an Islamic government.

One of the victims, Stephane Charbonnier, Charlie Hebdo's editor, had been the target of death threats and was living under police protection.

French officials have placed Paris on the highest security alert status in the wake of the attack.

Fr. Lombardi said the attack "ravaged the city of Paris" and was "sowing death, throwing consternation into the whole of French society" and "profoundly disturbing all peace-loving persons, well beyond the borders of France."

French Catholic leaders also condemned the terrorist act.

"This is obviously unspeakable terror. Nothing can justify such violence," the French bishops' conference said. "The barbarism expressed in these killings hurts us all."

"It affects more than freedom of expression, a fundamental element of our society," the bishops said, adding that French society consists of "diversity of all kinds" and "must continually work to build peace and brotherhood."

By coincidence, four French imams were at the Vatican for a two-day visit organized by the French bishops' conference. The attack took place while they were at Pope Francis' general audience at Vatican City's Paul VI Hall.

Two of the imams spoke to the French media outlets La Croix and I-Media, condemning the attack as a "hijacking" of Islam. They called on French Muslims to denounce the attacks and hold public demonstrations against them. The imams warned that the attacks could split society and harm the assimilation and integration of Muslims into French society.

The French bishops expressed "deep emotion and horror" at the attack and voiced their concern for the victims' families and friends. They expressed their "great sadness" for the editors and team of Charlie Hebdo.

They also warned that in such a situation anger "can overcome us" and people in France "need more than ever" to work to "strengthen peace."

Cardinal Andre Vingt-Trois of Paris expressed his "horror" and "deep compassion for the families and friends of the victims."

"With the Catholics of Paris, he condemns this act of barbarism and called more than ever to work to build relationships of peace and mutual respect in our society," a spokesman for the cardinal said.

U.S. leaders also reacted to the attacks. President Barack Obama said he "strongly" condemned the "horrific shooting."

"Our thoughts and prayers are with the victims of this terrorist attack and the people of France at this difficult time," he said. "Time and again, the French people have stood up for the universal values that generations of our people have defended. France, and the great city of Paris where this outrageous attack took place, offer the world a timeless example that will endure well beyond the hateful vision of these killers."

U.S. Secretary of State John Kerry said that free expression and a free press are "universal values" that can never be eradicated "because brave and decent people around the world will never give in to the intimidation and the terror that those seeking to destroy those values employ."

Niwad Awad, national executive director of the U.S.-based Council on American-Islamic Relations condemned the "brutal and cowardly attack" and repudiated any "assault on freedom of speech, even speech that mocks faiths and religious figures."

"The proper response to such attacks on the freedoms we hold dear is not to vilify any faith, but instead to marginalize extremists of all backgrounds who seek to stifle freedom and to create or widen societal divisions."

Awad offered his condolences to the families and loved ones of the attack's victims.