In the Legislative Assembly (Oct 1791–Sept 1792), the Jacobins, or the "radicals", sat on the left side of the chamber. They would later go on to establish a republican dictatorship. The Feuillants, a splinter group of the Jacobins, sat on the right; they were a relatively broad group that encompassed constitutional monarchists to ultra-royalists. About half of the chamber sat with neither in the middle, and were called the Marsh, or marais.
The term left and right appear to start being used in English to refer to broader political ideals rather than geographical location around 1820, according to the examples given in the OED.
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