  We show that relativistic strings of open and closed types in Minkowski
space-time of dimension 3 and 4 have topologically stable singular points. This
paper describes the structure of singularities, derives their normal forms, and
introduces a local characteristic of singularity (topological charge),
possessing a global law of conservation. Two other types of solutions (breaking
and exotic strings) are also considered, which have singularities at arbitrary
value of dimension.
