Thousands of New Yorkers donned "I Run for Boston" bibs during a 4-mile run Sunday in Central Park, one of a number of races held around the world in support of the victims of the marathon bombings.
"It was really quite a powerful morning," said Mary Wittenberg, CEO of the New York Road Runners.
Wittenberg said later Sunday at another run dedicated to victims that she had been in close communication with Boston Marathon organizers.
"This is one community," Wittenberg said. "After 9/11, we were all New Yorkers. After last Monday, we're all Bostonians. And I just want to add, I think now we've got one world of runners."
More than 6,000 runners took part in Sunday's City Parks Foundation Run for the Parks, which was planned before Monday's bombings.
Organizers turned it into a show of solidarity by selling "I Run for Boston" T-shirts with proceeds going to the One Fund Boston, the official fund for those affected by the bombing.
Other "Run for Boston" events have taken place around the U.S. and the world, with many runners wearing blue and yellow, the official Boston Marathon colors.
More than 500 runners gathered in St. Louis on Saturday for a Unity Run. In San Francisco, about 400 people ran 4 miles along the Embarcadero on Friday. A run christened "Boston Strong San Diego" is planned for Monday.
In Michigan, runners braved sub-freezing temperatures and a partly flooded course Sunday for the Lansing Marathon, which was dedicated to the Boston victims. In Burlington, Vt., a 5-kilometer walk-run on Saturday raised more than $10,000 for Massachusetts General Hospital's emergency fund and One Fund Boston.
In Greece, marathon runners wore black bracelets with the text "Thessaloniki to Boston" and several also wore a label on their running gear that read "Why?" There was a visible presence of extra security at the "Alexander the Great" marathon compared to previous races.
Sunday's London Marathon started with tributes to the Boston victims and a moment of silence. The London event was the first major race since the twin bombings in Boston killed three people and injured more than 180 others.
In New York, security for Sunday morning's run was tightened in response to the attacks.
Runners were told not to bring bags. Instead, clear plastic bags were provided by race officials for those who brought belongings.
The afternoon run, also in Central Park, was organized through an email chain among running groups and drew several hundred runners.
Among them was Elle Green, a New Yorker wearing a bright yellow shirt from Monday's Boston Marathon.
Green said she had just finished the marathon when she heard the first explosion.
"I saw a couple of people loaded into ambulances," she said. "It was pretty horrific."
Green said runners are "a special breed" who take care of one another on and off the course.
"It's such an innocent activity," she said. "It's just such a positive experience, and for something like that to happen, it tears at our hearts."