This paper is the first part of the proof of the conformal bootstrap for Liouville conformal field theory on surfaces with a boundary, devoted to Segal's axioms in this context. We introduce the notion of Segal's amplitudes on surfaces with corners and prove the gluing property for such amplitudes. The semi-group of half-annuli and its generator are studied and we develop the necessary material for proving its spectral decomposition using scattering theory in the companion paper \cite{GRW2}. The Segal gluing properties and the spectral decomposition allows us to prove the conformal bootstrap formula for correlation functions of Liouville conformal field theory with a boundary. This has several important applications to the study of conformal blocks (analyticity and convergence) in \cite{remypreprint}, in the construction of a unitary representation of mapping class group in the space of conformal blocks, and the study of random moduli \cite{ARSmoduliRPM} in Liouville quantum gravity.