U.S. President Barack Obama and his likely Republican challenger have suspended campaigning, out of respect for victims of the shooting rampage at a Colorado movie theater.

Twelve people were killed when the gunman opened fire during the premiere of the latest Batman movie in the Denver suburb of Aurora. About 60 others were wounded.

Mr. Obama called off a planned re-election rally and ordered flags to be flown at half staff. Republican Mitt Romney also suspended campaigning, calling the incident “a hateful act.”

The suspected shooter was arrested a short time after the rampage, and identified as 24-year-old James Holmes. The motive for the attack was not clear, but police said there was no indication of a link to any terrorist group.

Police said the man set off canisters spewing an irritant and then started randomly shooting. Witnesses said he fatally shot an infant at point-blank range.

Some said they initially thought the shooting was a special effect that was part of the opening night movie showing.

“At first I thought it was a part of the movie, that the speakers were extremely loud, but then there was the smoke behind me. It just didn't seem right.”

The movie theater attack was the worst mass shooting in the United States since 2009 when an Army psychiatrist killed 13 soldiers and civilians at a military base in Texas.

The suspected theater gunman told investigators he had multiple explosives at his residence in a North Aurora apartment building, and they later found the apartment booby-trapped with flammable and explosive materials. People at five nearby buildings were ordered out of their residences as authorities sought to disarm the dangerous site.