The goal was initially ruled out for an offside against Wayne Rooney but after consulting with his assistant, Clattenburg overturned the decision and gave the goal that earned United a point, to put them one point behind leaders Chelsea.
Ferguson said: "It's an obvious own goal and the referee had to allow it. I don't know what the linesman thought.
"It was clearly an own goal and the linesman flags for offside. I don't know what to say about that."
TV replays showed Rooney was just onside when Patrice Evra struck the initial shot that deflected into the net off Dann.
McLeish tries to quell Brum expectations
Birmingham manager Alex McLeish was less certain the goal should have stood but agreed the officials had eventually made the correct decision.
"The linesman's flagged for it and we felt that if he flagged for it then it shouldn't be given," he said.
"I thought at the time that it should have been given offside. We thought Rooney interfered with the guy who put it in the net.
"But he looked onside on the replay so they've ultimately reached the right decision but in the wrong way!"
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