  Collective behaviour of final-state hadrons, and multiparton interactions are
studied in high-multiplicity $ep$ scattering at a centre-of-mass energy
$\sqrt{s}=318$ GeV with the ZEUS detector at HERA. Two- and four-particle
azimuthal correlations, as well as multiplicity, transverse momentum, and
pseudorapidity distributions for charged-particle multiplicities $N_{\textrm
ch} \geq 20$ are measured. The dependence of two-particle correlations on the
virtuality of the exchanged photon shows a clear transition from
photoproduction to neutral current deep inelastic scattering. For the
multiplicities studied, neither the measurements in photoproduction processes
nor those in neutral current deep inelastic scattering indicate significant
collective behaviour of the kind observed in high-multiplicity hadronic
collisions at RHIC and the LHC. Comparisons of PYTHIA predictions with the
measurements in photoproduction strongly indicate the presence of multiparton
interactions from hadronic fluctuations of the exchanged photon.
