Al-Jazeera and others seem unwilling to call the "pro-Mubarak" demonstrators for what they are without more details. I think these "demonstrators'" methods speak rather clearly.
If they were counter-demonstrators, they would use the same tactics as the anti-government demonstrators. They would march, hold signs, and chant slogans. They would not come in on horses and camels and beat those of another opinion. They would not drop stones, cement blocks, and fire bombs from buildings.
Proving that these pro-Mubarak people are security police is another matter. But consider that the security police were outmaneuvered and outmanned by the anti-government demonstrators. Maybe they disappeared for a few days to regroup and come back in another guise.