Despite a call for a truce, the main opposition protest camp in the Yemeni capital Sanaa, known as Change Square, has come under heavy fire and sniper attack in the hours since Ali Abdullah Saleh, the president, returned.
Many are reported to have been killed in the attack.
The bloodshed reinforces fears that Saleh's return signals the beginning of a concerted effort to crush his rivals.
The US has conceded that his return was a surprise, urging him to initiate a full transfer of power and arrange for presidential elections before the end of this year.
With defecting units clashing with forces loyal to the regime, is Yemen heading for a civil war? And is there any mileage left in the Gulf's mediation efforts?
Inside Story discusses with guests: Bernard Haykel, a professor of Near Eastern Studies at Princeton University; Mohamed Qubaty, the spokesman for the opposition's National Yemeni Council; and Ameen Al Himyari, a Yemeni political analyst based in Qatar.