HONG KONG (MarketWatch) -- Amid a heavy market selloff, 203 mainland-China-traded companies announced separately Tuesday that trading in their shares had been suspended. This brought the total number of shares in trading halt over the past seven days to 651, or about 23% of the entire pool of 2,808 listed stocks, the Securities Daily reported Tuesday. Many of the companies didn't reveal the reasons behind the trading suspensions, though some cited reasons including the consideration of unspecified significant events, asset restructuring, or private share placements, the report said. Many market observers saw the exodus into trading halts as a way for companies to protect their stocks from the current sharp drop for Chinese markets, according to the report, with the Shanghai Composite Index SHCOMP, -0.63% having plunged 12.1% last week. Reuters reported separately that the companies would face fines if they were discovered to have requested trading suspensions without good reason. Among the 615 issues, 37% came from the Shenzhen Stock Exchange's Small and Medium-sized Enterprise board (SME), while more than 22% were from the ChiNext SZX00065, -1.03% of start-ups, according to the the Securities Daily.