  In this paper we report on the early evolution of core-collapse supernova
explosion following the birth of a magnetar with the dipolar magnetic field of
B=10^{15}G and the rotational period of 2ms, which was studied by means of
axisymmetric general relativistic MHD simulations. The numerical models exhibit
highly collimated magnetically-driven jets very early on. The jets are
super-Alfvenic but remain sub-fast until the end of the simulations (t=0.2s).
The power released in the jets is about 3x10^{50}erg/s which implies the
spin-down time of ~37s. The total rotational energy of the magnetar,
E~10^{52}erg, is sufficient to drive hypernova but it is not clear as to how
large a fraction of this energy can be transfered to the stellar envelope.
Given the observed propagation speed of the jets, v_p~0.17c, they are expected
to traverse the progenitor in few seconds and after this most of the released
rotational energy would be simply carried away by these jets into the
surrounding space. Our results provide the first more or less self-consistent
numerical model of a central engine capable of producing, in the supernova
setting and on a long-term basis, collimated jets with sufficient power to
explain long duration GRBs and their afterglows. Although the flow speed of our
jets is relatively low, v_j~0.5c$, the cooling of proto-neutron star will
eventually result in much higher magnetization of its magnetospheres and
ultra-relativistic asymptotic speeds of the jets. Given the relatively long
cooling time-scale we still expect the jets to be only weakly relativistic by
the time of break out. This leads to a model of GRB jets with systematic
longitudinal variation of Lorentz factor which may have specific observational
signatures both in the prompt and the afterglow emission.
