Seat for vehicles for the transport of passengers comprising a seat and a backrest, said seat and/or backrest comprising a support structure, comprising at least two metal arms substantially parallel to each other, and a sheet capable of being stretched between said arms, - said sheet being provided, at least at the side edges thereof, with first attachment means which allow the attachment thereof to said arms - said arms being in turn provided with second attachment means constrainable to said first attachment means (51, 53) of said sheet and being elastically connected to each other through elastic constraints, whereby said elastic constraints enable said arms to take: - a first "rest position" for said support structure in which they are tension-free; and - a second "attachment position" which is reachable starting from said first "rest position" by forcing said arms to a mutual approach thereof sufficient to allow the attachment of said first and second attachment means; characterised in that said elastic constraints comprise a cross member of said support structure, said cross member being joined to said arms to form a U-shaped support structure, and in that said elastic constraints further enable said arms to take: - a third "pretension position", intermediate between said attachment and rest positions, which is reachable by releasing said arms and at which the distance of the side edges of said sheet is such as to pretension it; and - a fourth "tension position" at which a tension can be applied to said sheet up to the maximum operating tension thereof, said "tension position" being reachable starting from said "pretension position" via a spacing means capable of spreading apart, moving them away relative to one another, said arms of said support structure of said seat and/or backrest. - said sheet being provided, at least at the side edges thereof, with first attachment means which allow the attachment thereof to said arms - said arms being in turn provided with second attachment means constrainable to said first attachment means (51, 53) of said sheet and being elastically connected to each other through elastic constraints, whereby said elastic constraints enable said arms to take: - a first "rest position" for said support structure in which they are tension-free; and - a second "attachment position" which is reachable starting from said first "rest position" by forcing said arms to a mutual approach thereof sufficient to allow the attachment of said first and second attachment means; characterised in that said elastic constraints comprise a cross member of said support structure, said cross member being joined to said arms to form a U-shaped support structure, and in that said elastic constraints further enable said arms to take: - a third "pretension position", intermediate between said attachment and rest positions, which is reachable by releasing said arms and at which the distance of the side edges of said sheet is such as to pretension it; and - a fourth "tension position" at which a tension can be applied to said sheet up to the maximum operating tension thereof, said "tension position" being reachable starting from said "pretension position" via a spacing means capable of spreading apart, moving them away relative to one another, said arms of said support structure of said seat and/or backrest. - a first "rest position" for said support structure in which they are tension-free; and - a second "attachment position" which is reachable starting from said first "rest position" by forcing said arms to a mutual approach thereof sufficient to allow the attachment of said first and second attachment means; - a third "pretension position", intermediate between said attachment and rest positions, which is reachable by releasing said arms and at which the distance of the side edges of said sheet is such as to pretension it; and - a fourth "tension position" at which a tension can be applied to said sheet up to the maximum operating tension thereof, said "tension position" being reachable starting from said "pretension position" via a spacing means capable of spreading apart, moving them away relative to one another, said arms of said support structure of said seat and/or backrest.