  A $t$-semiarc is a pointset ${\cal S}_t$ with the property that the number of
tangent lines to ${\cal S}_t$ at each of its points is $t$. We show that if a
small $t$-semiarc ${\cal S}_t$ in $\mathrm{PG}(2,q)$ has a large collinear
subset ${\cal K}$, then the tangents to ${\cal S}_t$ at the points of ${\cal
K}$ can be blocked by $t$ points not in ${\cal K}$. We also show that small
$t$-semiarcs are related to certain small blocking sets. Some characterization
theorems for small semiarcs with large collinear subsets in $\mathrm{PG}(2,q)$
are given.
