Mizuho Securities Co. resumed operations of its online trading service Thursday after suspending it for two and a half days due to a system failure.

The brokerage, which is part of Mizuho Financial Group Inc., said the previous day that it would restart the disrupted service by 6 a.m. Thursday, but operations did not return to normal until noon.

Mizuho Securities, which has 1.17 million online trading accounts, could not properly process about 10,000 buy or sell orders due to the system glitch. It plans to compensate customers for about 1,200 orders which could otherwise have been transacted.

The trouble began to affect orders Friday evening but was not discovered until early Monday morning, Mizuho Securities said. It continued to accept orders Monday but was forced to suspend the service the following day as unprocessed orders piled up.

The brokerage has said the problem was caused by a human error made when it updated the trading system. It has received about 470 complaints from customers since Tuesday.

On average 50,000 people trade in stocks and investment trusts each day through Mizuho Securities’ online system, according to the company.