By Scott Malone and Daniel Lovering
BOSTON (Reuters) - Wearing an orange prison jumpsuit, with his arm in a cast, accused Boston Marathon bomber Dzhokhar Tsarnaev pleaded "not guilty" to committing the worst mass-casualty attack on U.S. soil since 9/11, a crime that could bring him the death penalty.
Appearing in court for the first time, the 19-year-old ethnic Chechen - a naturalized U.S. citizen - spoke clearly, answering seven times that he was "not guilty" and occasionally glancing back at the gallery, where survivors and victims' relatives were watching.
Tsarnaev is charged with killing three people and injuring about 264 others by setting off homemade bombs - pressure-cookers filled with explosives, nails and ball bearings - assembled by him and his older brother, Tamerlan. Prosecutors say the brothers placed backpacks containing the bombs among the spectators near the finish line of the race on April 15.
Several days later, in the suburb of Watertown, a Massachusetts Institute of Technology police officer was killed in a shootout, during which 26-year-old Tamerlan Tsarnaev died after his brother ran over him with car as he escaped. The ensuing manhunt resulted in a day-long lockdown of most of the Boston area until Dzhokhar was found, badly wounded, hiding in a boat in a backyard.
Tsarnaev's appearance in the federal courtroom on Wednesday was the first time he has been seen in public since his arrest on April 19. His hair was long and unruly, his left arm in a cast and the orange jumpsuit, unbuttoned to the waist, revealed a black T-shirt underneath.
He fidgeted, scratched his face and looked around the courtroom, watching prosecutors as they spoke and occasionally looking back at about 30 survivors of the attack and victims' families.
"He didn't seem too shaken up by this. He didn't seem affected one bit, but I'm not a mind reader," said John DiFava, chief of the MIT police department, who attended the proceeding.
Tsarnaev is also charged in the shooting death of 27-year-old police officer Sean Collier.
The biggest challenge for Tsarnaev's attorney, public defender Miriam Conrad, will be sparing him the death penalty, observers said.
Security was tighter than usual on Wednesday outside Boston's U.S. District Courthouse, which is also the site of the ongoing murder and racketeering trial of mob boss James "Whitey" Bulger, now in its fifth week.
A handful of Tsarnaev supporters were outside and a few attended the proceeding.
"It was a little heartbreaking, but Dzhokhar and I have faith in Allah," said one supporter, Mary Churbuck, who wore a shirt with Dzhokhar's image and the slogan "Free the Lion."
"He's rolling with the punches," Churbuck said. "There's no evidence that he did do it. They don't have any evidence of him putting his backpack down."
According to court papers, Tsarnaev scrawled a note on an inside wall and beams of the boat in which he hid.
"The U.S. Government is killing our innocent civilians," the note read, according to the papers. "We Muslims are one body, you hurt one you hurt us all."
"Now I don't like killing innocent people it is forbidden in Islam but due to said it is allowed," he wrote, according to court papers. "Stop killing our innocent people and we will stop."
Three people died in the April 15 bombing - 29-year-old restaurant manager Krystle Campbell, graduate student Lingzi Lu, 23; and 8-year-old Martin Richard. MIT police officer Collier was killed three days later, according to the indictment.
Prosecutors said the government planned to call between 80 and 100 witnesses and that the trial would likely last three to four months. A status hearing was scheduled for September 23.
(Additional reporting by Stephanie Simon; Editing by Dina Kyriakidou and Gunna Dickson)