Events with no charged particles produced between the two leading jets are studied in proton-proton collisions at s=7 TeV . The jets were required to have transverse momentum pTjet>40 GeV and pseudorapidity 1.5<|ηjet|<4.7 , and to have values of ηjet with opposite signs. The data used for this study were collected with the CMS detector during low-luminosity running at the LHC, and correspond to an integrated luminosity of 8 pb-1 . Events with no charged particles with pT>0.2 GeV in the interval -1<η<1 between the jets are observed in excess of calculations that assume no color-singlet exchange. The fraction of events with such a rapidity gap, amounting to 0.5-1% of the selected dijet sample, is measured as a function of the pT of the second-leading jet and of the rapidity separation between the jets. The data are compared to previous measurements at the Tevatron, and to perturbative quantum chromodynamics calculations based on the Balitsky-Fadin-Kuraev-Lipatov evolution equations, including different models of the non-perturbative gap survival probability.