  A measurement of the production cross-section for top quark pairs($\ttbar$)
in $pp$ collisions at $\sqrt{s}=7 \TeV$ is presented using data recorded with
the ATLAS detector at the Large Hadron Collider. Events are selected in two
different topologies: single lepton (electron $e$ or muon $\mu$) with large
missing transverse energy and at least four jets, and dilepton ($ee$, $\mu\mu$
or $e\mu$) with large missing transverse energy and at least two jets. In a
data sample of 2.9 pb-1, 37 candidate events are observed in the single-lepton
topology and 9 events in the dilepton topology. The corresponding expected
backgrounds from non-$\ttbar$ Standard Model processes are estimated using
data-driven methods and determined to be $12.2 \pm 3.9$ events and $2.5 \pm
0.6$ events, respectively. The kinematic properties of the selected events are
consistent with SM $\ttbar$ production. The inclusive top quark pair production
cross-section is measured to be $\sigmattbar=145 \pm 31 ^{+42}_{-27}$ pb where
the first uncertainty is statistical and the second systematic. The measurement
agrees with perturbative QCD calculations.
