One fissure is 8 meters long and about 4 centimeters wide at some points, with about 5 cm drop from one side to the other.

Cracks also spread to adjacent flowerbeds and the sidewalk.

Some residents have speculated that the cracks were caused by land subsidence due to the high density of high-rise buildings in the area.

The information office of the Shanghai municipal government updated its official micro blog on Thursday, and said the underground structural engineering of Shanghai Tower was completed at the end of last year, and the factors that caused the subsidence have been eliminated.

Ge Qing, design director of Shanghai Tower Construction and Development, also wrote messages on Sina Weibo, confirming all was safe and sound during construction of the base of Shanghai Tower.

Engineering experts said such "settlement" cracks were common anywhere in the world and had nothing to do with the weight or height of buildings.

"Groundwater and rainfall may be blamed for settlements, and the soft soil foundation in Shanghai is another reason," said Liu Dongwei, chief architect of the China Institute of Building Standard Design and Research.

"So it is important to take foreseeable problems into consideration before construction and making safer design plans to prevent the large-scale emergence of cracks."

Dozens of skyscrapers, each taller than 100 m, gather in the Lujiazui area of the city, including the 101-story Shanghai World Finance Center and 88-story Jin Mao Tower.

Shanghai Tower is due for completion in 2014 when it will become the tallest building in China and the second tallest worldwide.