Egypt's ruling military council said it would not allow the continuation of "illegal" strikes and protests that have disrupted the country's economy.
In a statement on a state news agency, the military said it would "confront" such unrest with "legal steps".
The statement appears to be referring to the workforce strikes that have spread across the country since before Hosni Mubarak's fall from power a week ago.
It was not clear whether the warning also applied to the massive political protests centred on Cairo's Tahrir Square.
Protesters held a new massive rally there on Friday, demanding the military now listen to its demands for reform, and organisers said they would hold similar weekly rallies.
The groups that sparked the 18-day revolt that led to Mr Mubarak's downfall called the rally the "Friday of Victory and Continuation", a name that reflects both their pride in forcing a change in national leadership and their worries about the future.
The aim of the march was to keep the upbeat spirit of the earlier protests alive, and some flag-draped protesters in Cairo clapped or played musical instruments as they waited for prayers to begin.