Learn Hardcore Japanese: kinkyuu no baai ha keisatu ni denwa wo site kudasai. - In case of an emergency, please call the police.

Because 緊急 is a noun/na-adjective and it modifies another noun (場合) with の: 緊急の場合 = “the case of an emergency.” While dictionaries list 緊急 as a na-adjective, the natural collocation here is 緊急の, not 緊急な. You’ll commonly see 緊急の事態, 緊急の連絡, etc. “緊急な場合” is less idiomatic.

What does 場合は do here, and why は instead of が?

場合 means “case/situation.” 場合は marks that whole phrase as the topic/condition: “As for the case when …” Using が would incorrectly try to make it the subject. You’ll also see 場合には, which adds a touch of emphasis/delimitation but means the same thing here.

Can I use 時 or 際 instead of 場合? What’s the nuance?

Yes:

緊急の時(は): more everyday, “when there’s an emergency.”
緊急時(は): concise and formal; often in manuals/signage.
緊急の際(は): formal/polite written style.
緊急の場合(は): neutral-to-formal, emphasizes “in the event/if that happens.”

Why is は pronounced “wa” here?

When は is the topic particle, it’s pronounced “wa.” When it’s part of a word (花はな, 母はは), it’s “ha.” So 場合は is read “baai wa.”

How do you pronounce the whole sentence?

Kana: きんきゅうのばあいはけいさつにでんわをしてください。
Romaji: Kinkyū no baai wa keisatsu ni denwa o shite kudasai.

Why is に used after 警察? Could I use へ?

With phone calls, the recipient takes に: 人・組織に電話する (call someone/an organization). へ is possible in some “to/toward” senses, but with 電話 it’s uncommon. Use 警察に電話する as your default.

Why is there を before して? Do I have to include it?

電話をする is a “suru-verb” built from the noun 電話. In the request form, that becomes 電話をしてください. The を is optional with many suru-verbs:

警察に電話してください (very common, concise)
警察に電話をしてください (also correct; a bit more deliberate/formal) Both are fine; dropping を is very common in modern usage with ください.

Why して (te-form) and not する?

Requests use the pattern verb-て + ください. So you need the te-form して: “please do (phone).” Plain する cannot attach to ください.

What’s the difference between 電話する and 電話をかける?

They largely overlap in meaning (“to call”). 電話をかける literally means “to place a call.” All of these are natural:

警察に電話してください。
警察に電話をかけてください。 Tiny nuance: 電話する is the broader “to call”; 電話をかける focuses on the act of dialing/placing a call.

How polite is 〜てください? Are there stronger/softer options?

〜てください is the standard polite request. Variants:

Softer/formal: 〜ていただけますか／〜ていただけますでしょうか
Very formal/service-y: 〜てくださいませ (store announcements)
Casual: 〜て (e.g., 電話して), or 〜てね (friendly)
Command: 〜しろ／〜せよ／〜しなさい (strong/rough or directive tone) For public instructions, 〜てください is just right.

Should ください be written in kana or kanji (下さい)?

For requests, write it in kana: ください. Using 下さい is understood but tends to look old-fashioned or mixes with the main verb “to give” (下さる). Many style guides recommend kana for the request auxiliary.

Why say 警察 and not 警官 or おまわりさん?

You call the organization, not an individual officer. 警察 = “the police” (institution). 警官/おまわりさん refers to an officer. In emergencies, you normally contact the police via the emergency number, so 警察に電話する is the natural phrasing.

Is there a more idiomatic way to mention the emergency number?

Yes. In Japan:

Police: 110 → 110番してください／110番に通報してください
Fire/Ambulance: 119
Maritime emergencies (Coast Guard): 118 So a very natural instruction is: 緊急の場合は110番してください。

Is the spacing in the sentence normal for Japanese?

No. Spaces were added for learners. Normally it’s written without spaces: 緊急の場合は警察に電話をしてください。

Can I change the word order?

Keep the conditional topic up front for clarity:

Natural: 緊急の場合は、警察に電話してください。
You can move minor pieces (e.g., add お: 警察にお電話ください), but don’t separate 緊急の場合は from the start in this kind of instruction.

Is お電話ください okay here?

Yes. Adding the honorific お to 電話 is common and polite: 警察にお電話ください. You’ll also see お電話をください, though お電話ください is the set phrase many prefer. Both are understood and polite.