U.S. municipal bond yields blew through previous record lows on Friday as investors snapped up tax-free debt amid a global flight to safety after Britain voted to leave the European Union.

Yields on top-rated 10-year munis dropped 11 percent, or 17 basis points, to close at 1.36 percent, while 30-year yields fell 15 basis points to close at 2.08 percent, according to Municipal Market Data (MMD).

Those levels surpass previous low yields on June 16 of 1.42 percent and 2.13 percent, respectively, according to Thomson Reuters' MMD.

MMD data goes back to June 1981, when the yield on a AAA-rated 30-year bond was 10.20 percent. Bond prices move inversely to yields.

The $3.7 trillion muni market, most of which is investment-grade tax-exempt debt issued by states and cities, was already on a hot streak with no end in sight, with demand outstripping supply for months.

For 38 straight weeks, investors have poured money into muni bond funds with a net $30.4 billion added year-to-date. The week ended June 22 had the highest inflows in over three years at $1.4 billion, according to Lipper data.

Over the past 12 months, munis have returned 7.03 percent versus 5.18 percent for Treasuries, according to Barclays indices.

The "feeding frenzy" for muni bonds is unabated, said Bank of America Merrill Lynch research strategist Philip Fischer.

"Munis are very strongly bid and we've been bullish on the muni market for a long time. We remain bullish," he said, adding: "We expect we will have another new low in muni rates in the next couple of weeks and we'll probably get some profit taking then and rates will go back down before the end of the year."

Some have urged caution because even riskier muni bonds are now expensive.