CAIRO, April 15 (Reuters) - A bomb attack wounded two policemen in central Cairo on Tuesday, state television reported.

The unknown assailants targeted a traffic police checkpoint, state television said.

Since the army toppled Egypt's first freely-elected president, Mohamed Mursi of the Muslim Brotherhood, in July, Islamist militants have stepped up attacks on security forces, killing hundreds.

The violence has scared away investors, hammering the economy of the biggest Arab state.

An Islamist insurgency in the Sinai Peninsula which has defied an army crackdown has spread to Cairo and other cities as the country prepares for presidential elections next month.

Abdel Fattah al-Sisi, the general who overthrew Mursi, is expected to win the poll easily. (Writing by Michael Georgy; Editing by Ron Popeski, John Stonestreet)