phnom penh cambodia main opposition party block hold memorial ceremony wednesday victims 1997 grenade attack political rally tension run high arrest leader last week u.s. embassy urge citizens exercise caution prime minister hun sen government accuse unite state conspire opposition leader kem sokha charge treason potentially fan anti-american sentiment embassy announce stop issue visa cambodian foreign ministry officials kem sokha cambodia national rescue party cnrp plan hold ceremony memorial capital phnom penh mark attack least 16 people kill rally march 30 1997 organize former opposition leader sam rainsy similar memorials permit previous years around pchum ben festival cambodians pay respect decease ancestors try hold ceremony memorial stop mu sochua one cnrp deputy presidents tell reuters want confront authorities want show sign intimidation say add ceremony move nearby pagoda phnom penh city hall spokesman meet measpheakdey say pagoda right place ceremony party request monument deny opponents hun sen accuse arrest kem sokha crack independent media critics ahead general election next year could face tough electoral challenge 30 years power unite state western countries call kem sokha release draw anti-american rhetoric government u.s. embassy say specific warn security issue urge u.s. citizens show caution embassy announce stop issue visa senior foreign ministry officials separate dispute cambodia refusal continue accept cambodian citizens unite state want deport china cambodia big donor investor far stand hun sen government chinese friends like strong back continue help cambodia every circumstance foreign country break hun sen say facebook page wednesday return trip china cambodian opposition block hold memorial service