Majority of nationalities can apply for Iranian visas directly with the embassy.
However, generally visa application process takes around 2-3 weeks as the embassy has to first submit the request for the visa approval to the Iranian Ministry of of Foreign Affairs in Tehran. Once you have this, the embassy will start the visa application process.
Applying for the visa pre-approval online will take care of the first stage of the process, speed up the application process at the embassy and reduce the amount of time the embassy keeps your passport for.
Please note, the visa pre-approval is embassy specific. That means you'll have to state at the time of application where you'll be getting the actual visa - this cannot be changed once the code has been applied for. If you decide to get the visa from a different embassy you'll need to get a new pre-aproval.
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