Authorities are swarming to the finish line of the Boston Marathon to investigate reports of two loud explosions, according to wire and TV reports.

The successive booms went off shortly before 3 p.m. and there are reports of injuries.

“People are hurt, they’ve stopped the Boston Marathon,” a producer told an anchor on CNN Live via telephone.

Reporters and observers quickly took to Twitter to alert people about the explosions near Copley Square, yet warned that misinformation may be floating around.

The CNN producer said there is a large police presence near the site of the reported explosions and it looks like patients are being treated on scene.

More as it becomes available…

Update 1: 3 Dead 30 injured…

Two explosions went off near the finish line of the Boston Marathon on Monday as runners completed the race and thousands of people cheered them on. Witnesses told NBC News that there were widespread injuries, some severe.

Video from the finish line showed screams and an enormous cloud of white smoke, and about 20 seconds between the blasts. The Associated Press reported that bloody spectators were being carried to a medical tent that had been set up to care for tired runners.

Jackie Bruno, a reporter for New England Cable News, said on Twitter that she saw people’s legs blown off.

“Runners were coming in and saw unspeakable horror,” she said.

Massachusetts General Hospital said that it had four patients and was expecting more.

Serghino Rene, 30, who works in alumni relations, was volunteering near the finish line not long before the bombing but was a few blocks away when he heard the explosion.

“We saw two big puffs. I thought maybe it was fireworks. Then it went off again. And then all of a sudden we heard people crying and running away. It was a huge horde of people just running away,” he said. “We just got away from the scene and away from tall buildings. I’m scared and emotional. I’m wondering if there are people who I was volunteering with who were still there and got hurt.”

NBC affiliate WHDH said that storefront windows nearby were blown out.

Update 2: 2 killed, 23 injured as explosions rock Boston Marathon finish line

David L. Ryan/Globe Staff

The scene in Boston after reported explosions.

Two people were killed and 23 were injured this afternoon as two powerful explosions detonated in quick succession near the Boston Marathon finish line in Boston’s Back Bay section, turning a scene of athletic celebration into bloody chaos.

Blood and broken glass covered the sidewalks in the area where the blasts occurred in the city’s Back Bay section. Immediately after the blast, some of the wounded could be seen to have lost limbs.

Hundreds of police officers and firefighters descended on the scene immediately after the blasts, which happened just before 3 p.m., clearing the finish-line viewing stands. FBI and State Police have joined the investigation, officials said. City officials urged people to stay away from the area.

Bomb squads are searching the area, said Steve MacDonald, a spokesman for the Boston Fire Department. Officials said a third explosion heard around 4 p.m. was a “controlled blast” staged as part of bomb squad activity.

“There were two bombs that exploded near the finish line in today’s Boston Marathon. We are working with law enforcement to understand what exactly has happened,” the Boston Athletic Association, the organization that runs the marathon, a magnet for runners from all over the world, said in a statement.

At two spots on Boylston Street, scores of people fell and were injured. A Globe reporter at the scene could see that some had lost limbs; others had been knocked unconscious.

Update 3: Fox News is reporting that Boston police have found 2 more unexploded devices…