Flags on U.S. government buildings across the country will be flown at half-staff over the next three days to honor the victims of the Pittsburgh synagogue massacre.
President Donald Trump calls it "a mark of solemn respect for the victims of the terrible act of violence."
He also ordered flags lowered at all U.S. embassies, consular offices, military facilities, and Naval ships.
Interfaith vigils were held from coast-to-coast Sunday and in many Canadian cities. A number of National Football League games held moments of silence before kickoff.
The Vancouver Canucks professional hockey team also paused before their game with the Pittsburgh Penguins in Vancouver to remember the dead.
The Eiffel Tower in Paris was darkened Sunday night.
Authorities in Pittsburgh are calling what happened at the Tree of Life synagogue a "hate crime," saying suspected gunman Robert Bowers shouted anti-Semitic threats as he opened fire.
The 46-year-old Bowers faces 29 criminal charges. Documents outlining the allegations against him say Bowers was armed with an AR-15 assault rifle and three handguns. He said that he wanted all Jews to die because he believed Jews "were committing genocide to his people." That apparently refers to his belief that a Jewish refugee agency is helping foreign nationals enter the U.S. and that it endangers non-Jews in America.
In a message he apparently posted online just minutes before the attack, Bowers said the Hebrew Immigrant Aid Society, "likes to bring invaders in that kill our people. I can't wait while my people are getting slaughtered...I'm going in."
Officials said 11 people -- eight men and three women -- were killed. They ranged in age from 54 to 97. Six people were wounded, including four police officers, before Bowers was found barricaded inside the synagogue, shot, and arrested. He is recovering from his wounds.
The FBI said Bowers was not previously known to law enforcement, but apparently had posted a string of anti-Semitic threats online, particularly on the Gab.com website, where conspiracy theories are common.
Gab, which bills itself as the "free speech" alternative to Twitter and Facebook, has become a popular place to post content unwelcome or prohibited on other platforms. Gab responded with a statement Sunday:
"We refuse to be defined by the media's narratives about Gab and our community. Gab's mission is very simple: to defend free expression and individual liberty online for all people."
On top of Bowers' page, one quote said, "Jews are the children of Satan," according to screenshots of the now-suspended account released by the SITE Intelligence Group, which tracks extremist views.
Police had responded to emergency calls about 9:45 a.m. Saturday morning as regular religious services were being held by three congregations at the Tree of Life Synagogue. Witnesses said that Bowers, as he entered the synagogue, shouted, "All these Jews must die!"
Authorities said they found victims at three locations inside the synagogue, located in Pittsburgh's Squirrel Hill neighborhood. The local medical examiner, Dr. Karl Williams, said, "Lots of shots were fired, there were casings everywhere."
The Anti-Defamation League, which has tracked hatred and violence against Jews since the 1970s, said the mayhem is "likely the deadliest attack on the Jewish community in the history of the United States."
"We are devastated," said the group's head, Jonathan Greenblatt. "Our hearts break for the victims, their families, and the entire Jewish community."
World leaders denounced the attack, deploring it as an affront to humanity.
President Trump told a political rally late Saturday, "This evil, anti-Semitic attack is an assault on all of us. We must stand with our Jewish brothers and sisters to defeat anti-Semitism and vanquish the forces of hate."
On Twitter, Trump said, "All of America is in mourning over the mass murder of Jewish Americans at the Tree of Life Synagogue in Pittsburgh. We pray for those who perished and their loved ones, and our hearts go out to the brave police officers who sustained serious injuries. This evil Anti-Semitic attack is an assault on humanity. It will take all of us working together to extract the poison of Anti-Semitism from our world. We must unite to conquer hate."
Former U.S. President Barack Obama said, "We grieve for the Americans murdered in Pittsburgh. All of us have to fight the rise of anti-Semitism and hateful rhetoric against those who look, love, or pray differently. And we have to stop making it so easy for those who want to harm the innocent to get their hands on a gun."
Pope Francis at the Vatican called the massacre an "inhuman act of violence." He prayed "to help us to extinguish the flames of hatred that develop in our societies."
In Israel, Prime Minister Benjamin Netanyahu said, "It is hard to overstate the horror of the murder of Jews who congregate on the Sabbath and who were murdered only because they were Jews. On my behalf, on behalf of the government of Israel and the nation of Israel I convey our heartfelt condolences to the families that have lost dear ones. We all pray for the speedy recovery of the wounded."
German Chancellor Angela Merkel called the assault an act of "blind anti-Semitic hatred," while United Nations Secretary General Antonio Guterres called for a united world effort "to roll back the forces of racism, anti-Semitism, Islamophobia and other forms of racism."