  The majority of stars form in clusters. Therefore a comprehensive view of
star formation requires understanding the initial conditions for cluster
formation. The goal of our study is to shed light on the physical properties of
infrared dark clouds (IRDCs) and the role they play in the formation of stellar
clusters. This article, the first of a series dedicated to the study of IRDCs,
describes techniques developed to establish a complete catalogue of Spitzer
IRDCs in the Galaxy. We have analysed Spitzer GLIMPSE and MIPSGAL data to
identify a complete sample of IRDCs in the region of Galactic longitude and
latitude 10deg < |l|<65deg and |b|<1deg. From the 8micron observations we have
constructed opacity maps and used a newly developed extraction algorithm to
identify structures above a column density of N_{H2} > 1x10^{22} cm^{-2}. The
24micron data are then used to characterize the star formation activity of each
extracted cloud. A total of 11303 clouds have been extracted. A comparison with
the existing MSX based catalogue of IRDCs shows that 80% of these Spitzer dark
clouds were previously unknown. The algorithm also extracts ~ 20000 to 50000
fragments within these clouds, depending on detection threshold used. A first
look at the MIPSGAL data indicates that between 20% and 68% of these IRDCs show
24micron point-like association. This new database provides an important
resource for future studies aiming to understand the initial conditions of star
formation in the Galaxy.
