Tokyo stock markets successfully reopened on Friday after a hardware failure caused an unprecedented day-long halt to trade on one of the world's biggest exchanges. The benchmark Nikkei 225 index was up 0.52 percent or 121.63 points at 23,306.75 in early trade while the broader Topix index was up 0.49 percent or 7.93 points at 1,633.42. Late Thursday, Tokyo Stock Exchange officials apologised for the shutdown, blaming a hardware malfunction that they said had now been fixed. The problem was discovered before Thursday's opening bell and meant the entire trading day was lost on Tokyo's two leading indexes, as well as smaller exchanges in Nagoya, Fukuoka and Sapporo. Officials said there was no indication of a cyberattack, and the problem had been traced to a memory breakdown that failed to properly trigger a switch to a back-up system. Fixing it would have required a system restart that "would have created confusion among investors and market participants", said TSE president Koichiro Miyahara at a Thursday press conference. "After discussing with market participants, we decided to stop the market for the whole day." "We caused great inconvenience to many market participants and investors... We sincerely apologise," he added. Officials said the faulty hardware had been replaced and personnel would be deployed to monitor the system and avoid a repeat. kh-sah/jah