A strong explosion of unknown origin shook the busy shopping street of Istiklal in the Turkish city of Istanbul on Sunday, leaving four people dead and wounding dozens more, officials said. Police cordoned off the area, where crowds were dense on Sunday afternoon, and helicopters were flying over the city centre as sirens were sounding. “I was 50–55 metres [yards] away, suddenly there was the noise of an explosion. I saw three or four people on the ground,” witness Cemal Denizci, 57, told AFP. “People were running in panic. The noise was huge. There was black smoke. The noise was so strong, al…