SANAA (Reuters) - Yemen's opposition rejected on Thursday an offer to join Gulf-mediated talks on a transfer of power in the Arabian peninsula state, and set a two-week deadline for President Ali Abdullah Saleh to step down. The opposition said the Gulf Arab mediation offer, which was to have included talks in Saudi Arabia as early as Saturday, was not clear enough on how fast a transition would take place, even after a request for clarification from Gulf ambassadors. "We have renewed our emphasis on the need for speeding the process of (Saleh) standing down to within two weeks. Therefore we will not go to Riyadh," said Mohammed al-Mutawakkil, a prominent opposition leader. Gulf foreign ministers, trying to ease the threat that Yemeni instability could pose to the region, had invited Saleh and his opponents to talks on a transfer of power to end a political standoff that risks devolving into violence. Saudi and Western allies of Yemen fear that a prolonged standoff in Yemen, where Saleh has faced two months of protests demanding his overthrow, could ignite clashes between rival military units and cause chaos that would benefit an active al Qaeda wing operating in the poor, mountainous country. Saleh has accepted the Gulf framework for talks, but the opposition has seesawed. It first rejected the Gulf offer, citing the lack of a transition timeframe and complaining it appeared to offer Saleh a waiver from prosecution. Opposition figures then met the ambassadors of Saudi Arabia, Oman and Kuwait Tuesday seeking clarification of the Gulf Cooperation Council's understanding of a "transfer of power," with some hinting that talks could start as early as Saturday. Mutawakkil, however, said the opposition could reach an agreement on granting assurances against prosecution, leaving the timing of a transfer as the major holdup. "We didn't find in the clarifications that the ambassadors presented anything that meets our demands for an immediate removal," Mutawakkil said. "There was nothing new from the Gulf Cooperation Council ambassadors." A Saudi foreign ministry spokesman had no comment, nor did an official for the 6-member Gulf Cooperation Council. Saleh, who has already lost control of several provinces, has warned of civil war and the break-up of Yemen if he is forced to step aside before organising parliamentary and presidential polls over the next year. More than 116 protesters have been killed in clashes with security forces since late January, and there are fears the violence could escalate in the impoverished country, half of whose 23 million people own a gun. "This is proof that the opposition doesn't want dialogue or peaceful solutions, but want to come to power through chaos," Tarek al-Shami, a spokesman for Yemen's ruling party, said of the opposition decision to turn away from the talks. The government also accused forces belonging to defected General Ali Mohsen, whose troops have been protecting the protesters in Sanaa, of abducting two government soldiers from an area near the city's main protest zone. Mohsen, a kinsman of Saleh, has welcomed the Gulf mediation but did not object to the opposition refusal. A source in Mohsen's office denied seizing the soldiers. The drawn out standoff over a transition, however, has not appeared to affect the energy of street protesters who turned out for two straight days by the tens of thousands in the capital Sanaa despite driving rain. "The opposition's stance is good and brings them closer to the street. We ask them to reject any assurances to the president and his family," said Radwan al-Abasi, a youth protest leader in the Red Sea coastal town of Hudaidah. A transition of power in Yemen could technically last until the next presidential election scheduled for 2013, a prospect the opposition finds unacceptable. Saleh has offered new parliamentary and presidential elections this year as part of political reforms, but says he should stay in power to oversee the change or hand over to what he calls "safe hands." While protesters want Saleh out now, some in the opposition, which includes Islamists, leftists and Arab nationalists, are prepared for him to stay in power for several months more before handing over to his vice-president. But nearby countries worry that Saleh is an obstacle to stability in a country that overlooks a shipping lane used to transport over 3 million barrels of oil a day. Even before the start of the protests, inspired by the toppling of the Tunisian and Egyptian presidents, Saleh was struggling to quell a separatist rebellion in the south and cement a truce with Shia rebels in the north.