SANAA (Reuters/AFP) - At least 20 protesters were killed in the Yemeni capital Sanaa on Sunday, when security forces opened fire on one of the biggest demonstrations against President Ali Abdullah Saleh in recent months, a Reuters witness said. Scores were wounded when protesters tried to break through lines of police who also sprayed them with tear gas as tens of thousands spilt out of Change Square, where many youths have camped out since early this year demanding an end to Saleh's 33-year rule. Injured protesters were rushed into a field hospital in the square on stretchers. "This is the worst day I've seen in three months. We're expecting more dead to come in," said doctor Jamal al-Hamdani, who was treating patients. A Reuters witness saw dozens of men slumped on the ground, overcome by tear gas inhalation. Men on motorbikes and ambulances whisked them away from the scene. The Defence Ministry said on its website protesters threw petrol bombs, setting a police car ablaze. State media blamed gunmen belonging to opposition parties for opening fire on the march. Meanwhile, Yemen's rival political parties were divided on Sunday over a stalled Gulf-brokered power transfer deal which a senior Saudi official said President Ali Abdullah Saleh's deputy will sign within a week. "To end the crisis we must agree with the opposition on a mechanism for implementing the Gulf initiative," a spokesman for Saleh's ruling General People's Congress (GPC) party, Tariq al-Shami, told AFP. "This is why dialogue is necessary" with the opposition, which is demanding that Saleh sign the deal before any discussion on ways to implement it, Shami said. "Dialogue before transfer of power is out of the question," said key opposition leader Sultan al-Atwani. The so-called Gulf Initiative was proposed by the six-nation Gulf Cooperation Council (GCC) and sets the path for a peaceful transition of power. The presidential decree gave Hadi "the necessary constitutional authority" to endorse the plan and to negotiate a power transfer mechanism with the opposition. A high-level Saudi official told AFP on Saturday that Hadi will sign the initiative "within a week" after the United States on Thursday said that this is what it had "hoped" for. Saleh, who has ruled Yemen since 1978, has been recovering in Saudi Arabia from a June 3 attack on his presidential compound, but has refused to hand power to his deputy or sign the Gulf Initiative.