By Peter Salisbury and William Maclean SANAA/DUBAI (Reuters) - The Houthi rebels who stunned the Arab world with the sudden seizure of Yemen's capital will have to strive to cement their power in the face of well-armed rivals, a test of strength that could tip the unstable country deeper into turmoil. A suicide bombing last Thursday that killed 47 in Sanaa is an example of the indiscriminate tactics Yemenis fear some armed groups are prepared to employ to check the ascent of the Shi'ite Muslim movement that swept into the city on Sept 21. The attack was claimed by al Qaeda in the Arabian Peninsula (AQAP), apparently furious at the ability of Houthi fighters to take over Yemen's fragile state and dictate terms to its embattled President Abd-Rabbu Mansour Hadi. The mood in Sanaa is apprehensive.