The charred 28-story building surrounded by scaffolding in downtown Shanghai had fresh flowers on all sides Sunday as citizens flocked in to mourn at least 58 people killed in a blaze here six days ago.

A black banner was put up on Jiaozhou Road, where the apartment building is located, with white Chinese characters reading "In sad memory of those who died in the deadly blaze of Nov. 15".

Thousands of people arrived at the site early Sunday morning to mourn victims of the deadliest fire in China's largest city in decades. Family members and friends wept. Strangers stood in silent tribute.

On a clearing in front of the building, some mourners spelt out "11.15" -- the date of the disaster -- with white chrysanthemums and candles.

Sunday is the seventh day since the disaster, a day the Chinese believe the dead would revisit their families before leaving for good. The tradition is for family members to prepare a feast, and burn a paper ladder along which the dead would climb to heaven.

Shanghai's Communist Party chief Yu Zhengsheng, mayor Han Zheng and other senior officials joined the mourning crowd, bowed three times and presented white chrysanthemums.

The blaze erupted Monday afternoon after sparks from welding equipment set nylon construction netting and bamboo scaffolding alight.

Luo Lin, head of the State Administration of Work Safety, has blamed the tragedy on unlicensed welders, illegal multi-layered sub-contracting and poor management.

Shanghai police have detained 12 people in connection with the fire.