Play was postponed, a group of children narrowly avoided injury from flares hurled onto the pitch, and the fire brigade was called as crowd trouble again marred Canberra's premier soccer competition.

The flares were hurled from outside McKellar Park onto the ground in the second half, halting play for several minutes. A second flare burned outside the ground after the smoke cleared.

The incident occurred despite a pre-finals warning from Capital Football to clubs on Thursday night that the organisation would come down hard on crowd trouble.

There was no additional security at the ground, but Capital Football representatives said guards would be on duty at today's minor semi-final at Deakin Stadium between Canberra FC and Canberra Olympic - a rematch of last year's grand final.

Capital Football conceded it was caught off guard after last year's decider when a small band of drunk and abusive fans brandishing flares marred FC's 8-5 win over Olympic.

Football Federation Australia this week also vowed to clamp down on crowd trouble following several incidents in A-League pre-season matches.

Yesterday's incident overshadowed Cooma's 2-1 win over Belconnen United.

The Tigers inflicted the Blue Devils with just their second loss this season, earning next weekend off and their first grand final appearance in six years.

In a lacklustre opening half, it took 28 minutes before a shot was taken, an own goal for Cooma eventually breaking the drought in the 42nd minute.

United's Dustin Wells hit the crossbar with a header in the opening minute of the second stanza and missed another chance to the near side five minutes later.

It let Cooma off the hook and allowed it to equalise when Alex Oloriegbe crossed for Goran Josifovski to head home.

After the disruption of the flares, Belconnen missed some more chances, the closest from Milan Popovich, who lobbed advancing keeper Francis Bangweni, narrowly missing an open goal.

It was a brilliant drive from Josifovski through traffic which eventually put Cooma in front, the same player at the centre of a mid-season administrative error which cost the Tigers vital competition points providing redemption in the 83rd minute.

''The federation tried to take it away from us when we lost the points in the season,'' Tigers coach Gaby Wilk said.

''But we still had the strength to keep going and get something from it. The things we win are on the park, not in the office.''

Belconnen coach Steve Forshaw was left ruing his side's missed opportunities, but paid due respect to Cooma, the only team to have beaten it this season.

''All credit to [Cooma] they stuck to their guns, they had to get out of a hole and they did - their second goal was a good strike,'' he said.

''We could have done without [the loss], but we had chances in the second half where we could have put the game to bed and didn't take them and ultimately that's cost us.''

Belconnen will face the winner of today's minor semi-final between FC and Olympic for a chance to meet Cooma again in the grand final.