PHNOM PENH, Cambodia — Riot police fired smoke grenades and water cannons on Sunday at rock-throwing opposition supporters in the capital who vowed to continued their protests against the government until a post-election deadlock was resolved. Connect With Us on Twitter Follow @nytimesworld for international breaking news and headlines. Twitter List: Reporters and Editors At least one police officer was injured. Most of Phnom Penh was calm, however, after about 20,000 protesters gathered at a city park earlier in the day to keep up their push for an independent investigation of the results from the election in July. The opposition says the ballot was marred by irregularities that robbed it…