During normal operation even below 10,000ft the aircraft will be pressurised. This pressurisation occurs at the start of the takeoff roll and until landing on any pressurised aircraft I've flown.
Only if there's a pressurisation system failure and there is no pressurisation will it be potentially possible to open the doors but there are still many reasons that this wouldn't be easy to accomplish.
For example, on the 737 the doors are hinged at the front and would need to swing into the airflow to open so this would be impossible at any speed, however they could be cocked ajar but I doubt it would be possible to open them enough for anything or anybody to squeeze out. The overhangs are locked closed when the aircraft is in flight irrespective of pressurisation.
On some aircraft, opening the cabin doors was part of the smoke removal process. For example on the 747, see the SAA 295 crash it was mentioned.