ISTANBUL (AFP) - A bomb blast near a bus stop in central Istanbul injured eight people Thursday, officials said, amid mounting jitters ahead of Turkey's June 12 elections. The bomb exploded in Etiler, an upscale district in Turkey's largest city, blowing out the rear windows of a public bus and wounding both passengers inside and passers-by in the street, among them a policeman. The blast also shattered the windows of several vehicles and a nearby store, an AFP reporter at the scene said. "A bomb with a medium impact, placed on an electric bike, caused the blast at 08:58 am (0588 GMT)," Istanbul police chief Huseyin Capkin said in televised remarks. Capkin said no suspects had been identified, but the fact that there was a police school nearby increased suspicions that police were the target of the attack. Eight people aged between 23 and 55, among them three women, were injured and two of them were in critical condition, the Istanbul health directorate said in a statement carried by Anatolia news agency. A woman had a foot torn off and another suffered burns in her respiratory system, a senior health official said, according to Anatolia. No one has claimed responsibility for the attack so far, Istanbul Governor Huseyin Avni Mutlu said, pointing at "a high possibility" that a time bomb caused the blast. "No attack can shake our nation's unity or its strong standing and morale in the face of terrorism... The perpetrators will not go unpunished," he said. Several outlawed armed groups - Kurdish, Islamist and extreme leftist - have carried out bomb attacks in Istanbul in the past. The last incident was in October when a suicide bomber wounded 32 people in an attack on a police patrol, which was claimed by a radical Kurdish group. Earlier this month, the Kurdistan Workers' Party (PKK), which has been waging a separatist war against Ankara for 26 years, claimed responsibility for an attack on an election convoy of the ruling party in northern Turkey that killed a policeman. The ambush was followed by a warning from jailed PKK leader Abdullah Ocalan that "all hell will break loose" unless Turkey commits itself to full-fledged negotiations to end the conflict by June 15.