After Wednesday's 2-2 tie, Toronto now has to win, settle for another 2-2 draw and win in overtime or on penalty kicks, or engineer an even higher-scoring tie in next week's second leg in Los Angeles to move on to the CONCACAF Champions League semifinals.
Donovan scored in the 89th minute off a goalmouth scramble from a Beckham corner to spoil the home fans' party under the dome.
Until then, it had looked like Toronto was going to hang on, having scored early and survived a series of defensive wobbles against a Los Angeles team that has only recently been able to field all its top players at the same time.
"We were unlucky and we were the better team," said Toronto coach Aron Winter, who meant every word of it.
It was frenetic and entertaining soccer, if not the beautiful game before a boisterous crowd of 47,658 at the Rogers Centre. It was also fun on a big stage, with a big star in Beckham orchestrating the visitors' attack.
Ryan Johnson and rookie Luis Silva scored for Toronto, which led 2-0 after just 18 minutes.
Mike Magee cut the lead to 2-1 in the 29th minute for the defending MLS champions. And then came the last-minute heroics.
Both defences were in pre-season form, making for lots of scoring chances. The second half was particularly sloppy.
Los Angeles coach Bruce Arena was appalled at the artificial turf, calling it dry, difficult, slippery "and a little bit dangerous."
"Give our guys a lot of credit," said Arena. "This is a game where you can come up with a bunch of excuses not to get any kind of result ... And they hung in there and showed a lot of character.
"The point (from the tie) is huge because going back to L.A. next Wednesday, we're in a decent position. Not that it's perfect, but it's not bad and it took a lot of character to come back down two goals under these conditions. Our guys weren't pleased with the field conditions in the early going, you could see that."
Toronto seemed to do just fine with the conditions, however.
The star-studded Galaxy were ripped apart on defence by the searing pace of Toronto's Joao Plata.
It was a corner caused by Plata that led to the first goal in the 12th minute.
A Los Angeles defender headed the Torsten Frings inswinger away but the ball went straight to Johnson, who sent it high into the goal with his left foot.
Johnson and his teammates then headed to the corner for a wild celebration as Los Angeles players looked at each other.
"We just got a little bit sloppy," Arena lamented.
Silva made it 2-0 in the 18th, rising to send a looping header past Saunders after a long-range cross from Frings. Silva, a rookie who is from Los Angeles and went to school at University of California, Santa Barbara, sank to his knees and crossed himself.
"Our start was fantastic," said Winter.
The Toronto defence was nowhere to be seen in the 29th minute when Magee trimmed the lead, heading in a Sean Franklin cross from the byline.
There was a long wait for the final goal.
"I thought there were a number of goals in this game for us ... I was confident we could get another goal," said Arena.
The Galaxy coach eased up on the field complaints as his news conference wore on — "Both teams played on the same field," he conceded. And he gave the Toronto crowd full marks.
"That was terrific support of their team. No question about it," he said. "It's a huge statement for this franchise."
Said Winter: "The atmosphere was fantastic."
Winter conceded the pitch "was not normal."
"But still, even with that pitch, we played well."
Neither side seemed happy with the work of Jamaican referee Courtney Campbell. Toronto striker Danny Koevermans pulled up his shorts in the second half to show an ugly welt after a clash with goalie Josh Saunders.
The second leg is scheduled for March 14 at the Home Depot Center in Carson, Calif., with the winner advancing to play either the Seattle Sounders or Mexico's Santos Laguna.
The evening started quietly but had ramped up nicely for kickoff.
Beckham drew the loudest ovation during Galaxy introductions, with boos and cheers almost even.
The crowd, quiet before the game, found its voice for the Toronto introductions with a rousing welcome in particular to Frings, Koevermans and the diminutive Plata.
As at BMO Field, the south end of the stadium led the way in enthusiasm. It looked like the bottom stand had been daubed in red paint and fans there launched the TFC chant as kickoff approached.
Even the Toronto subs were treated like royalty as they came on the field. Winter, wearing a stylish suit that made him look like he was playing a gig with The Jam later in the evening, was also warmly welcomed.
Confetti streamed out of the stands as the teams marched in to suitably pompous music.
The 36-year-old Beckham found himself surrounded by streamers when he tried to take a corner in the third minute. The part-time underwear model looked like he had come out of a Christmas cracker.
After a second-half corner, Beckham had streamers wrapped around his body as he raced back up the field. Just before the tying goal, he held up a can as more debris was launched at him from the stands as he prepared to take the corner.
A team official said the Galaxy planned to complain to CONCACAF about the can.
The former England captain also had a slow start but began to go in search of the ball, launching long-range passes down the field as Los Angeles looked to unlock the Toronto defence.
Wednesday night's game was played indoors because of concerns over the weather, although Toronto was treated to a balmy afternoon high of 15 and it was nine degrees at kickoff.
The chance to sell extra tickets thanks to Beckham and the Galaxy was also a factor, no doubt. Tickets ranged from $15.50 to $77.75.
Toronto started Stefan Frei in goal with the rock-steady Frings in front of him at the heart of the backline.
Their job was to stop Beckham, Donovan and Robbie Keane, the Galaxy stars who made US$12.2 million in salary between them last season, according to the MLS Players' Union.
The final eight of the tournament features three teams from MLS, four from Mexico and one from El Salvador.