Around 3,000 Muslim Brotherhood activists took the oath, raising their right hands during a protest in Amman on Friday.
King Abdullah II said during talks this week with President Nicolas Sarkozy of France that reforms are "coming soon."
Bowing to pressure, Abdullah has said he will allow prime ministers to be chosen through parliamentary elections rather than appointing them himself. But he says the change will take two to three years.
The government also promised wider freedoms.
But critics remain suspicious of the current premier, an ex-army general widely seen incapable of introducing reforms.