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JWST observations indicate a surprising excess of luminous galaxies at $z\sim10$ and above, consistent with efficient conversion of the accreted gas intostars, unlike the suppression of star formation by feedback at later times. Weshow that the high densities and low metallicities at this epoch {\itguarantee} a high star-formation efficiency (SFE) in the most massivedark-matter haloes. Feedback-free starbursts (FFBs) occur when the free-falltime is shorter than $\sim 1$ Myr, below the time for low-metallicity massivestars to develop winds and supernovae. This corresponds to a characteristicdensity of $\sim 3\times 10^3$cm$^{-3}$. A comparable threshold density permitsa starburst by allowing cooling to star-forming temperatures in a free-falltime. The galaxies within $\sim 10^{11} M_\odot$ haloes at $z \sim 10$ areexpected to have FFB densities. The halo masses allow efficient gas supply bycold streams in a halo crossing time $\sim 80$ Myr. The FFBs gradually turn allthe accreted gas into stars in clusters of $\sim 10^{4-7} M_\odot$ withingalaxies that are rotating discs or shells. The starbursting clouds areinsensitive to radiative feedback and are shielded against feedback fromearlier stars. We predict high SFE above thresholds in redshift and halo mass,where the density is $10^{3-4}$cm$^{-3}$. The $z\sim 10$ haloes of $\sim10^{10.8} M_\odot$ are predicted to host galaxies of $\sim 10^{10} M_\odot$with SFR $\sim 65 M_\odot$ yr$^{-1}$ and sub-kpc sizes. The metallicity is$\leq 0.1 Z_\odot$ with little gas, dust, outflows and hot circumgalactic gas,allowing a top-heavy IMF but not requiring it. The compact galaxies withthousands of young FFB clusters may have implications on reionization,black-hole growth and globular clusters.

Efficient Formation of Massive Galaxies at Cosmic Dawn by Feedback-Free Starbursts