Molecular-dynamics simulations of monatomic liquids interacting via a modified Johnson potential have been carried out to investigate the structure of liquids and the microscopic mechanism of the glass transition. The structure of liquids is described in terms of spherical harmonic representations of topology of local clusters and the orientational correlation among them. The variation of the averaged topology of the nearest-neighbor clusters exhibits an anomalous behavior at temperatures between the glass transition temperature (

T_{g}

) and a temperature much above

T_{g}

. This anomalous behavior is shown to be caused by aggregation of the clusters with icosahedral topology. The number of the icosahedral clusters increases progressively with decreasing temperature, and, at

T_{g}

, the increase is abruptly arrested. Within the icosahedral aggregates, the strong mirror-related orientational correlation exists with a correlation length growing over 10 Å near the glass transition temperature. The percolation of the icosahedral clusters and the mirror-related orientational order leading to the glass transition are discussed.

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