This seat is reserved.
'Riservato' — ri-ser-VA-to. Stress on the third syllable.
Use this to inform someone that the seat they are about to take is a priority seat (for the elderly, pregnant, or disabled), or to hold a seat for someone.
'Riservato' is the past participle of 'riservare' used as an adjective. It means reserved or set aside. The phrase without a preposition implies reserved in a general sense; 'riservato a + noun' specifies for whom: 'riservato agli anziani' (reserved for the elderly).
Questo posto è per i disabili.
This seat is for people with disabilities.
More explicit statement of who the seat is for.
Stia attento, questo è un posto prioritario.
Be careful, this is a priority seat.
Warning someone politely before they sit.
Potrebbe cedere il posto?
Could you give up your seat?
Polite request to someone sitting in a priority seat.
Priority seats in Italian metros are marked with stickers showing a pregnant woman, an elderly person, a parent with a child, and a person with a disability. Giving up your seat is a social expectation, not just a courtesy.