The phrase "up in arms" means to be angry or outraged about something. There are several synonyms that can be used in place of "up in arms" to convey a similar meaning. For example, you could say that people are "in an uproar" over something, which means they are protesting or expressing their displeasure. You could also say that they are "furious," "incensed," "outraged," or "livid" about the situation. All of these words suggest a high level of anger and a strong desire to take action. When looking for synonyms for "up in arms," consider the tone you want to convey and choose a word that fits that tone.