Our correspondent said prosecutors had not struck a deal with Shahzad in return for his guilty plea.
Some of the charges carry a mandatory life sentence.
"It's a war. I am part of the answer to the US terrorising the Muslim nations and the Muslim people"
Times Square bomb suspect
During Monday's hearing US District Judge Miriam Goldman Cedarbaum challenged Shahzad repeatedly with questions such as whether he had worried about killing children in the attack.
"One has to understand where I'm coming from," he replied. "I consider myself… a Muslim soldier."
The 30-year-old described his effort to set off the bomb saying he chose the warm Saturday night because it would be crowded with people he could injure or kill.
He said he conspired with the Pakistan Taliban, which provided more than $15,000 to fund his operation.
He explained that he packed his vehicle with three separate bomb components, hoping to set off a fertilizer-fuelled bomb packed in a gun cabinet, a set of propane tanks and gas canisters rigged with fireworks to explode into a fireball.
|Investigators say that had the bomb gone off it would have killed many people [Reuters]
He also said he was carrying a folding assault rifle for "self-defence".
After parking the SUV Shahzad said he lit a fuse and waited for between two-and-a-half to five minutes for the bomb to go off.
"I was waiting to hear a sound but I didn't hear a sound… So I walked to Grand Central and went home," he said.
According to witness reports, the bomb apparently sputtered, emitting smoke that attracted the attention of a nearby street vendor, who notified police, setting in motion a rapid evacuation of the area.
Shahzad was arrested two days later at New York's JFK airport minutes before the plane he had boarded was about to take off for Dubai.
Questioned by the judge during Monday's one hour hearing Shahzad dismissed the questions about the possibility that children would have died had the bomb gone off by saying the US did not care when children were killed in Muslim countries.
"It's a war. I am part of the answer to the US terrorising the Muslim nations and the Muslim people," he said.
"On behalf of that, I'm revenging the attack. Living in the United States, Americans only care about their people, but they don't care about the people elsewhere in the world when they die."
In a statement released after Monday's hearing, Eric Holderm, the US Attorney General, said Shahzad had "plotted and launched an attack that could have led to serious loss of life, and today the American criminal justice system ensured that he will pay the price for his actions."
Pakistan has arrested at least 11 people since the attempted attack although no one has been formally charged.
In the US three men in Massachusetts and Maine suspected of supplying money to
Shahzad have been detained on immigration charges and one of the suspects was recently transferred to New York.