The policemen were travelling in a minivan when the assailants in a pickup truck blocked their path and sprayed the vehicle with automatic rifle fire, the ministry said.
Jihadists, including Islamist State group militants, have killed hundreds of policemen and soldiers in attacks, mostly in the Sinai Peninsula and also in and around Cairo.
Egyptian criminal gangs have also killed policemen in shoot outs, but the attack bore the hallmarks of jihadists who have waged an insurgency since the military overthrew Islamist president Mohamed Morsi in 2013.
The interior ministry said the dead included a lieutenant and seven lower ranking policemen who were patrolling the area just south of the capital when they were ambushed late at night.
Militants had struck before in Helwan, killing a policeman standing guard outside a museum in June 2015.
The jihadists, who are based in the sparsely populated Sinai Peninsula bordering Israel and the Palestinian Gaza Strip, have repeatedly tried to make inroads in the capital, where police have had more success in quelling them than in Sinai.
— Hasan Sari (@HasanSari7) May 8, 2016
They have claimed several attacks in Cairo, including an attempted assassination of the interior minister in late 2013 and the bombing of the Italian consulate in July 2015.
More recently militants have conducted hit and run attacks on policemen in Cairo and small scale bombings.
– Retaliation –
They often claim their attacks are in retaliation for a bloody police crackdown on Islamist supporters of Morsi, which has killed hundreds of protesters and imprisoned thousands.
They have also targeted foreigners.
In October, the Islamic State group claimed responsibility for bombing a Russian airliner carrying holidaymakers from a south Sinai resort, killing all 224 people on board.
The group said it smuggled explosives concealed in a soda can on to the plane in airport at Sharm El-Sheikh, a popular Red Sea resort in south Sinai.
That attack prompted Russia to suspend all flights to Egypt, and has lost the country hundreds of millions of dollars in tourism revenues.
The bombing came two months after they abducted a Croatian oil worker near Cairo and beheaded him.
Police later tracked down the top Islamic State group operative in Cairo, who was linked to the Croat’s murder, and killed him in a shoot out.
But efforts to quell the insurgency in Sinai have floundered despite a massive army campaign.
In March, Islamic State gunmen killed 15 policemen in an attack on a checkpoint near the El-Arish, the provincial capital of North Sinai.
Since pledging allegiance to the Islamic State group, which controls parts of Syria and Iraq, in November 2014, the Sinai branch’s attacks have grown more sophisticated.
The military says it has killed more than 1,000 militants, occasionally publishing pictures of their bodies.
The claims are difficult to verify, with reporters having little access to the north of the peninsula.