But authorities said the panic might have first been sparked outside the tunnel when some revellers tried to jump over a barrier and fell, Wolfgang Rabe, the head of the crisis unit set up by Duisburg city authorities, said.
Authorities did not immediately cancel the festival, which police said drew about 1.4 million people, due to fears that an abrupt halt could spark a second panic.
A 'horror situation'
Juergen Kieskemper, the police commissioner in Duisburg, described the situation as "very chaotic".
Ambulances rushed to pick up victims, though rescue workers reportedly had trouble reaching some of the wounded. Police also tried to resuscitate a number of people inside the festival.
"Young people came to party. Instead, there was death and injury"
Angela Merkel, the German chancellor
"There was no escape," one Love Parade participant named Marius told Germany's Bild newspaper. "People were pressed into the wall. I was afraid I'd die."
Thomas Muenten, a reporter with Germany's ZDF television network, called it a "horror situation".
Angela Merkel, the German chancellor, said she was "aghast" at the stampede.
"In these difficult hours, my thoughts are with the relatives of the victims. They have my sympathy and my condolences," she said in a statement.
"Young people came to party. Instead, there was death and injury. I am aghast and saddened by the sorrow and the pain."
The festival was once held in Berlin, but has been held in western Germany since 2007.
Stefan Wlach, a German journalist at the festival, told Al Jazeera that authorities had not managed the annual event as well as had been done in other cities.
"Before the festival started, [authorities] told us everything was organised. But as you see now, nothing was organised and it all ends in total chaos," he said.
"In former times, the Love Parade ... went well. In the other cities, you had lots of space, you had big streets and avenues where the Love Parade was held. Here you have a small ... industrial area.
"Everybody who can count should have known that you don't get 1.4 million people in this area. It's too small."
The original Berlin Love Parade grew from a 1989 peace demonstration into
a huge outdoor celebration of club culture that drew about 1.5 million people
at its peak in 1999.