At least 45 pro-government troops have been killed in a suicide car bombing claimed by the Islamic State group in the southern Yemen city of Aden, officials said.
The soldiers were preparing to travel to Saudi Arabia to fight Houthi rebels in Yemen's north.
The men were at a staging area near two schools and a mosque where they were registering to join the expedition.
The Saudis hope to train up to 5,000 fighters and deploy them to the Saudi cities of Najran and Jizan, near the border, Yemeni security officials said.
More than 60 wounded were being taken to three area hospitals, they added.
Aid group Doctors Without Borders reported on social media that their hospital in Aden had received 45 dead, while the Yemeni officials earlier put the figure at 25 but said it was likely to rise.
The IS-run Aamaq news agency said the attack was carried out "by a fighter from the Islamic State who targeted a recruitment centre".
Yemen is embroiled in a civil war pitting the internationally recognised government and a Saudi-led coalition against the Shiite Houthi rebels, who are allied with army units loyal to a former president. The fighting has allowed al Qaida and an Islamic State affiliate to expand their reach, particularly in the south.
The UN and rights groups estimate that at least 9,000 people have been killed since fighting escalated in March 2015 with the start of Saudi-led air strikes targeting the Houthis and their allies. Some three million people have been displaced inside the country, the Arab world's poorest.
UN-mediated peace talks in Kuwait were suspended earlier this month with no signs of progress.
The Houthis and forces allied to former president Ali Abdullah Saleh seized Yemen's capital, Sanaa, in September 2014, forcing the internationally recognised government to flee the country.
The Saudi-led campaign against the Houthis has pushed them out of southern Yemen, but has failed to dislodge them from Sanaa and the rest of the north.