  This is the first paper of a series aimed at studying the chromosphere of
active binary systems using the information provided for several optical
spectroscopic features. Simultaneous H{alpha}, Na I D{1}, D{2}, and He I D{3}
spectroscopic observations are reported here for 18 systems. The chromospheric
contribution in these lines have been determined using the spectral subtraction
technique. Very broad wings have been found in the subtracted H{alpha} profile
of some of the more active stars. These profiles are well matched using a
two-components Gaussian fit (narrow and broad) and the broad component could be
interpreted as arising from microflaring. Prominence-like extended material
have been detected in a near-eclipse H{alpha} observation of the system AR Lac.
The excess emission found in the Na I D{1} and D{2} lines by application of the
spectral subtraction technique and the behaviour of the H{alpha} line in the
corresponding simultaneous observations indicate that the filling-in of the
core of these lines is a chromospheric activity indicator. For giant stars of
the sample the He I D{3} line has been detected in absorption in the subtracted
spectra. An optical flare has been detected in UX Ari and II Peg through the
presence of the He I D{3} in emission in coincidence with the enhancement of
the H{alpha} emission.
