  We explain the observed multiwavelength photon spectrum of a number of BL Lac
objects detected at very high energy (VHE, $E \gtrsim 30$ GeV), using a
lepto-hadronic emission model. The one-zone leptonic emission is employed to
fit the synchrotron peak. Subsequently, the SSC spectrum is calculated, such
that it extends up to the highest energy possible for the jet parameters
considered. The data points beyond this energy, and also in the entire VHE
range are well explained using a hadronic emission model. The ultrahigh-energy
cosmic rays (UHECRs, $E\gtrsim 0.1$ EeV) escaping from the source interact with
the extragalactic background light (EBL) during propagation over cosmological
distances to initiate electromagnetic cascade down to $\sim1$ GeV energies. The
resulting photon spectrum peaks at $\sim1$ TeV energies. We consider a random
turbulent extragalactic magnetic field (EGMF) with a Kolmogorov power spectrum
to find the survival rate of UHECRs within 0.1 degrees of the direction of
propagation in which the observer is situated. We restrict ourselves to an RMS
value of EGMF, $B_{\rm rms}\sim 10^{-5}$ nG, for a significant contribution to
the photon spectral energy distribution (SED) from UHECR interactions. We found
that UHECR interactions on the EBL and secondary cascade emission can fit
gamma-ray data from the BL Lacs we considered at the highest energies. The
required luminosity in UHECRs and corresponding jet power are below the
Eddington luminosities of the super-massive black holes in these BL Lacs.
