CAIRO — An explosion at an election rally for presidential front-runner Abdel-Fattah el-Sissi injured four people, including two policemen, state media reported Sunday. The candidate was not at the rally.El-Sissi, who left the military two months ago to run for president, has made virtually no campaign appearances, with aides citing security concerns. The former military leader told a TV interviewer this month that there had been two attempts on his life, but gave no details.By contrast, el-Sissi’s sole opponent, liberal politician Hamdeen Sabahi, has been barnstorming the countryside. While he has drawn appreciative crowds at rallies, he is seen as having little chance to win the May 26-27 election.The explosion at the al-Sissi rally in Cairo, apparently caused by a homemade device, took place late Saturday, Egypt’s official news agency reported. There was no immediate claim of responsibility. The country has been hit by a wave of attacks, most aimed at security forces, in the 10 months since the military-backed interim government took power. Islamic militant groups have claimed responsibility for most of them.El-Sissi led the July coup that deposed Islamist Mohamed Morsi, Egypt’s first democratically elected president. The army moved against Morsi after huge nationwide protests demanding his ouster.