  The ATLAS detector at CERN's Large Hadron Collider (LHC) will be exposed to
proton-proton collisions from beams crossing at 40 MHz. At the design
luminosity of 10^34 cm^-2 s^-1 there are on average 23 collisions per bunch
crossing. A three-level trigger system will select potentially interesting
events in order to reduce the read-out rate to about 200 Hz. The first trigger
level is implemented in custom-built electronics and makes an initial fast
selection based on detector data of coarse granularity. It has to reduce the
rate by a factor of 10^4 to less than 100 kHz. The other two consecutive
trigger levels are in software and run on PC farms. We present an overview of
the first-level trigger system and report on the current installation status.
Moreover, we show analysis results of cosmic-ray data recorded in situ at the
ATLAS experimental site with final or close-to-final hardware.
