Securities regulators here in China are issuing a call for calm amid the massive sell-off on the mainland markets this past week.

Friday's trading saw both the Shanghai and Shenzhen exchanges lose over 6-percent in value.

Through this past week, Shanghai finished down 13-percent, the biggest weekly drop in 7-years.

Most market observers are attributing the sell-off on both Friday and the day before to an over-correction by investors worried about an over-heated market in Shanghai, which has jumped by around 140-percent over the past year.

However, a number of investors are pointing the finger at regulators, suggesting the lack of publicizing a change in the way in which trade volumes are calculated is to blame.

The China Securities Regulatory Commission brought in changes the week before which excluded certain transactions from being included in the overall volume totals, which some investors say spooked the market after traders eventually recognized the reduced totals.

However, Deng Ge with the CSRC says the suggestion doesn't make much sense, given the adjustment was brought in during the previous trading week.

"After the readjustment on June 10th, the Shenzhen Component Index actually rose for three days in a row. Investors generally bought stocks without a lot of selling. We're hoping investors carefully review market information and invest rationally."

Nine new companies began trading on the mainland markets on Friday.

New issues often drain money from the existing shares, as traders move money into the new issues from their existing portfolios.