The KM3NeT detector is a Cherenkov neutrino telescope being installed at two sites in the Mediterranean deep-sea. Its infrastructure will comprise several thousands of multi-PMT digital optical modules arranged in three-dimensional spatial arrays. The first two detection lines of KM3NeT are taking data since December 2015 and May 2016 respectively. These lines are the first of 2x115 KM3NeT/ARCA strings optimized for the detection of high-energy neutrino sources. Each line is about 700 m high and hosts 18 DOMs equally spaced by 36 m.
Results of the first detection lines are presented, including in-situ calibration and validation of the nanosecond timing capabilities, comparison with data from optical flashers, and the reconstruction of the first atmospheric muons. The results highlight the excellent photon detection capabilities of the KM3NeT multi-PMT technology and signify an important verification of the design.