Algerians USM were heading for a shock Group B win at the 90,000-seat Borg El Arab Stadium near Alexandria after taking a 30th-minute lead through Farouk Chafai.
But in the fourth minute of stoppage time, captain Zemmamouche dropped a cross at the feet of grateful Zambian Emmanuel Mayuka, who scored from close range.
Former Southampton striker Mayuka had come off the bench as 2016 runners-up Zamalek sought a way back into a game from which they were expected to collect maximum points.
Chafai briefly abandoned his defensive duties to sneak into the box for a free-kick, get goal-side of a defender and score off the post from point-blank range.
Salvaging a draw kept Zamalek top of the table with five points, USM and Al Ahly Tripoli of Libya have four and CAPS United of Zimbabwe three.
Friday, Ahly Tripoli scored three goals in nine minutes during the first half to shock CAPS 4-2 in Harare and become contenders for a top-two finish and a quarter-final place.
Zanaco of Zambia maintained a great start in Group D as an 85th-minute Diddy Kitumbo goal delivered a 2-1 win over struggling Coton Sport of Cameroon in Lusaka.
All the goals came from headers with Saith Sakala giving the hosts an early lead that Moussa Souleymanou cancelled out midway through the first half.
Zanaco have seven points, record eight-time champions Al Ahly of Egypt four, Wydad Casablanca of Morocco three and Coton none.
Ahly host 2016 semi-finalists Wydad late on Sunday in Alexandria needing a victory to regain first place on goal difference.
Ferroviario Beira of Mozambique won for the first time in Group A, edging Sudanese visitors Al Merrikh 1-0 through a 67th-minute Dayo Antonio goal.
It was a worthy match-winner as Antonio outran one defender and took the ball across another before firing into the net from close range.
Ferroviario are competing in the elite African club competition for the first time — 93 years after the club was formed.
A miserable match for Merrikh on a sub-standard Beira pitch was completed a minute from time when Ala Eldin Yousif was red-carded after a second booking.
Etoile Sahel of Tunisia, who host Al Hilal of Sudan late on Saturday, have six points, Ferroviario four, Hilal two and Merrikh one.