An unverified image published by local media showed a bearded man wearing a bulky ammunition vest sprawled on a street.
Reports suggest a second militant involved in the attack managed to escape.
A further five security guards were injured in the shocking rampage, according to the state-run news agency MENA.
Cairo’s security chief Khaled Abdel Al and a number of security officials have headed to the scene of the attack.
Loud sirens were heard in central Cairo as the incident was first reported at around 11am local time (9am UK time).
The attacker opened fire outside the Church of Great Martyr Mina
No group has yet claimed responsibility for the attack, but ISISterrorists have killed dozens of Christians across Egypt over the past year.
ISIS claimed a suicide bombing of a Cairo church in December 2016 followed by bombings of two churches north of the capital in April.
A further five security guards were injured in the shocking rampage
Egypt imposed a state of emergency following the church attacks and shootings.ISIS has been waging a deadly insurgency based in the Sinai peninsula bordering Israel and the Gaza Strip, killing hundreds of policemen and soldiers.
The army has poured in thousands of troops backed with armour and jets in a bid to crush the jihadists, but attacks have continued.