Opposition parties' announcement on Sunday to join protesters marks the second major setback in two days for President Ali Abdullah Saleh, a key U.S. ally.
On Saturday, two powerful chiefs from his own tribe abandoned him, and anti-government protesters mustered the largest crowds yet.
The mainstream opposition parties had been reluctant to join the protests, preferring instead a wait-and-see strategy.
However, on Sunday they said they would hold rallies Tuesday to show solidarity with the protesters. In recent weeks, Yemen has seen daily protests, inspired by successful uprisings in Egypt and Tunisia.