Stocks fell again on Friday - by more than 4%

The investors rioted after they were prevented from selling their shares as markets fell heavily.

Five people were arrested as hundreds threw stones and damaged cars.

The Securities and Exchange Commission says it is investigating the share fall and whether it was manipulated by bigger brokers.

Tyres burned

The BBC's Idrees Bakhtiar in Karachi said large numbers of police were on duty at the Karachi Stock Exchange, checking all identities.

Reforms have largely checked the chances of manipulation... but there is still room for further improvement Mohammad Sohail, Jahangir Siddiqui Capital Markets

The KSE 100 index touched an all time high of 10,305 last week but has plummeted since and continued to decrease on Friday.

Stocks fell more than 4% on Friday, closing down 349.15 points at 7,964.95. More than 108.52m shares were traded.

Securities and Exchange Commission officials say they are checking possible insider trading and market manipulation by big brokerage houses.

But they say it is routine practice to check any abnormal activities.

Mohammad Sohail, director of Jahangir Siddiqui Capital Markets, told the AFP news agency: "Reforms have largely checked the chances of manipulations and it is not being done at the levels of say 10 years ago, but there is still room for further improvement."

Thursday's protests also hit the Lahore exchange, where tyres were burned.

Both exchanges suspended trading in some of the main blue chip shares after they had dropped 6% or more.

Small investors were incensed that they were not able to sell their shares.

When Pakistani shares broke through the psychologically important 10,000 level, it was interpreted as a sign of economic recovery.