2000 Suzuki Vitara Breaks Suddenly Activate

While traveling at normal city speed (25-40mph) the breaks suddenly activates slowing down the vehicle to a complete alt. In less that a minute from the initial engine loss of power -rpm don't go higher- the breaks engage and stop the vehicle. A light smell of burning is noticed from the front breaks. The break pedal gets stiff and does not move under the foot pressure,
Waiting 5-10 minutes the break pedal regain/return to the normal foot reaction and the vehicle can travel normally again.
What can this be caused by?
2000 SUZUKY Grand Vitara 125Kmiles very well kept serviced by Firestone