The Shanghai Composite Index plunged 5.33 per cent, or 169.71 points, to 3,016.70 on turnover of 286.4 billion yuan ($43.6 billion).
The Shenzhen Composite Index, which tracks stocks on China's second exchange, tumbled 6.60 per cent, or 130.62 points, to 1,848.10 on turnover of 377.8 billion yuan.
The Shanghai index fell nearly 10 per cent last week, slammed by concerns over China's flagging economy, weakening currency and a regulatory blunder.
On Friday, authorities reversed course and suspended a new "circuit breaker" mechanism that had fuelled a global rout by twice automatically closing Chinese markets early in just four days.
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"The market is already in a downward spiral and it's still exploring the bottom after last week's plunge," Zhang Yanbing, an analyst at Zheshang Securities, told AFP.
"The economy remains weak and there's no driver for a market rebound."
Global investors have been alarmed by slowing growth in the world's second-largest economy, which is expected to have expanded last year at its slowest pace in a quarter of a century. Official data on fourth-quarter and annual growth is due to be released later in January.
On Monday, the People's Bank of China set the daily reference rate -- around which the yuan can move up or down two per cent -- at a stronger 6.5626 to USD 1.0.