SANA'A: Yemen's opposition warned the government Thursday that violence against street protesters demanding the ouster of President Ali Abdullah Saleh could derail a draft deal to end the political standoff. Plainclothes gunmen killed 10 people and wounded dozens more in Yemen's capital on Wednesday when they opened fire on anti-government marchers just days before a Gulf-mediated deal to resolve the crisis was due to be sealed. “In the event of your inability to protect protesters, we will find ourselves unable to pursue an agreement that the regime seeks to use to shed more blood,” the opposition coalition said in a statement. A deal providing for Saleh to be eased out within a month was expected to be signed Sunday in Riyadh, three months after Yemenis hit the streets to demand his departure, inspired by revolts that toppled autocratic rulers in Egypt and Tunisia. After weeks of violence, military defections and political reversals, the balance of power has tipped in recent weeks against Saleh, long a key ally of the West against Al-Qaeda. Wednesday's killings capped a day of demonstrations by tens of thousands of Yemenis, many protesting against the plan supported by the government and the main opposition group that would give Saleh a month-long window to resign. The Sana'a protesters were trying to reach an area beyond the district where they have been camped out since February, and were demanding Saleh leave office immediately, witnesses said. In addition to the 10 killed in Sana'a, a protester and a soldier died in clashes during protests in the southern port city of Aden Wednesday. Washington and neighboring oil producer Saudi Arabia want the political stalemate resolved. They fear a descent into more bloodshed in the Arabian Peninsula state would offer more room for a Yemen-based al Qaeda wing to operate. The tentative accord, brokered by the six-member Gulf Cooperation Council, would also give Saleh and his family and aides immunity from prosecution. It provides for Saleh to appoint a prime minister from the opposition, who would then form a transition government ahead of a presidential election two months after his resignation. But the one-month leeway allowed for Saleh to resign has sparked fears it may offer time for potential sabotage. Protesters have vowed to stay in the streets until their demands are met, and it was not clear that opposition parties, including Islamists and leftists, could halt the street unrest even if required to by the transition agreement. Around 140 protesters have been killed as unrest has swept Yemen, where some 40 percent of its 23 million people live on $2 a day or less, and a third face chronic hunger.