The public are invited to send messages of love to firstname.lastname@example.org, tweet them with the hashtag #loveisallaround or text the word love and a 120-character message to a designated number.
The messages go live from today (9 February). Messages that must be held until Valentine's Day itself should be emailed to BT.
Messages will be screened to ensure no inappropriate messages are shown and people will be notified if their message is due to appear on the BT Tower.
Suzi Williams, group marketing and brand director at BT, said the brand is also targeting women, who can, according to tradition, propose to men this year, as it is a leap year.
There are 22 BT Big Screens around the country in locations including Belfast, Birmingham, Cardiff, Edinburgh, Manchester and Plymouth.
Follow Matthew Chapman at @mattchapmanUK